It is Yudhisthira because when he was the crown prince, people were very happy with him and he was chosen the Yuvaraja because of his firmness, frankness, and honesty.
Vaisampayana continued, 'After the expiration, O king, of a year from this, Dhritarashtra, moved by kindness for the people, installed Yudhishthira, the son of Pandu, as the heir-apparent of the kingdom on account of his firmness, fortitude, patience, benevolence, frankness and unswerving honesty (of heart). And within a short time Yudhishthira, the son of Kunti, by his good behaviour, manners and close application to business, overshadowed the deeds of his father.
Bhima received education from Drona and further education from Balarama also due to which he became very strong
And the second Pandava, Vrikodara, began to receive continued lessons from Sankarshana (Valarama) in encounters with the sword and the mace and on the chariot. And after Bhima's education was finished, he became in strength like unto Dyumatsena himself and continuing to live in harmony with his brothers, he began to exert his prowess.
And Arjuna became celebrated for the firmness of his grasp (of weapons), for his lightness of motion, precision of aim, and his proficiency in the use of the Kshura, Naracha, Vala and Vipatha weapons, indeed, of all weapons, whether straight or crooked or heavy. And Drona certified that there was none in the world who was equal to Arjuna in lightness of hand and general proficiency.
"One day, Drona, addressing Arjuna before the assembled Kaurava princes, said, 'There was a disciple of Agastya in the science of arms called Agnivesa. He was my preceptor and I, his disciple. By ascetic merit I obtained from him a weapon called Brahmasira which could never be futile and which was like unto thunder itself, capable of consuming the whole earth. That weapon, O Bharata, from what I have done, may now pass from disciple to disciple. While imparting it to me, my preceptor said, 'O son of Bharadwaja, never shouldst thou hurl this weapon at any human being, especially at one who is of poor energy. Thou hast, O hero, obtained that celestial weapon. None else deserveth it. But obey the command of the Rishi (Agnivesa). And, look here, Arjuna, give me now the preceptorial fee in the presence of these thy cousins and relatives.' When Arjuna, on hearing this, pledged his word that he would give what the preceptor demanded, the latter said, 'O sinless one, thou must fight with me when I fight with thee.' And that bull among the Kuru princes thereupon pledged his word unto Drona and touching his feet, went away northward. Then there arose a loud shout covering the whole earth bounded by her belt of seas to the effect that there was no bowman in the whole world like unto Arjuna. And, indeed, Dhananjaya, in encounters with the mace and the sword and on the chariot as also with the bow, acquired wonderful proficiency.
Sahadeva obtained the whole science of morality and duties from (Brihaspati) the spiritual chief of celestials, and continued to live under the control of his brothers.
And Nakula, the favourite of his brothers taught by Drona, became known as a skilful warrior and a great car-warrior (Ati-ratha).
Pandavas and Arjuna became very powerful that they defeated powerful kings whom even Pandu could not defeat.
Indeed, Arjuna and the other Pandava princes became so powerful that they slew in battle the great Sauvira who had performed a sacrifice extending over three years, undaunted by the raids of the Gandharvas. And the king of the Yavanas himself whom the powerful Pandu even had failed to bring under subjection was brought by Arjuna under control. Then again Vipula, the king of the Sauviras, endued with great prowess, who had always shown a disregard for the Kurus, was made by the intelligent Arjuna to feel the edge of his power. And Arjuna also repressed by means of his arrows (the pride of) king Sumitra of Sauvira, also known by the name of Dattamitra who had resolutely sought an encounter with him. The third of the Pandava princes, assisted by Bhima, on only a single car subjugated all the kings of the East backed by ten thousand cars. In the same way, having conquered on a single car the whole of the south, Dhananjaya sent unto the kingdom of the Kurus a large booty.
Thus did those foremost of men, the illustrious Pandavas, conquering the territories of other kings, extend the limits of their own kingdom.
~ Mahabharata: Adi Parva: Sambhava Parva
Then the citizens, beholding the son of Pandu possessed of accomplishments, began, O Bharata, to speak of them in all places of public resort. And assembled in courtyards and other places of gathering, they talked of the eldest son of Pandu (Yudhishthira) as possessed of the qualifications for ruling the kingdom. And they said, 'Dhritarashtra, though possessed of the eye of knowledge, having been (born) blind, had not obtained the kingdom before. How can he (therefore) become king now? Then Bhishma, the son of Santanu, of rigid vows and devoted to truth, having formerly relinquished the sovereignty would never accept it now. We shall, therefore, now install (on the throne) with proper ceremonies the eldest of the Pandavas endued with youth, accomplished in battle, versed in the Vedas, and truthful and kind.
~ Mahabharata, Book 1: Adi Parva: Jatugriha Parva
This was before Lakshagraha where people planned to make Yudhisthira the king which made Duryodhana and Shakuni to form the conspiracy of Lakshagraha to burn the Pandavas, his own brothers for a kingdom.
Pandavas were divine as they were sons of gods and as soon as they were born, divine voices said that they would be virtuous, strong and intelligent who were born on auspicious days.
Then the handsome Kunti was united (in intercourse) with the god of justice in his spiritual form and obtained from him a son devoted to the good of all creatures. And she brought his excellent child, who lived to acquire a great fame, at the eighth Muhurta called Abhijit, of the hour of noon of that very auspicious day of the seventh month (Kartika), viz., the fifth of the lighted fortnight, when the star Jyeshtha in conjunction with the moon was ascendant. And as soon as the child was born, an incorporeal voice (from the skies) said, 'This child shall be the best of men, the foremost of those that are virtuous. Endued with great prowess and truthful in speech, he shall certainly be the ruler of the earth. And this first child of Pandu shall be known by the name of Yudhishthira. Possessed of prowess and honesty of disposition, he shall be a famous king, known throughout the three worlds.'
This was during the birth of Yudhisthira.
The god of wind thereupon begat upon her the child afterwards known as Bhima of mighty arms and fierce prowess. And upon the birth of that child endued with extraordinary strength, an incorporeal voice, O Bharata, as before, said, 'This child shall be the foremost of all endued with strength.'
This was during birth of Bhima.
'I shall give thee, O king, a son who will be celebrated all over the three worlds and who will promote the welfare of Brahmanas, kine and all honest men. The son I shall give thee will be the smiter of the wicked and the delight of friends and relatives. Foremost of all men, he will be an irresistible slayer of all foes.' Thus addressed by Vasava (the king of the celestials), the virtuous king of the Kuru race, well-recollecting those words, said unto Kunti, 'O fortunate one, thy vow hath become successful. The lord of the celestials hath been gratified, and is willing to give thee a son such as thou desirest, of superhuman achievements and great fame. He will be the oppressor of all enemies and possessed of great wisdom. Endued with a great soul, in splendour equal unto the Sun, invincible in battles, and of great achievements, he will also be extremely handsome. O thou of fair hips and sweet smiles, the lord of the celestials hath become gracious to thee. Invoking him, bring thou forth a child who will be the very home of all Kshatriya virtues.'
So Indra himself said he will give an excellent child who will be ever invincible in battle.
"Vaisampayana continued, 'The celebrated Kunti, thus addressed by her lord, invoked Sakra (the king of the gods) who thereupon came unto her and begat him that was afterwards called Arjuna. And as soon as this child was born, an incorporeal voice, loud and deep as that of the clouds and filling the whole welkin, distinctly said, addressing Kunti in the hearing of every creature dwelling in that asylum, 'This child of thine, O Kunti, will be equal unto Kartavirya in energy and Siva in prowess. Invincible like Sakra himself he will spread thy fame far and wide. As Vishnu (the youngest of Aditi's sons) had enhanced Aditi's joy, so shall this child enhance thy joy. Subjugating the Madras, the Kurus along with the Somakas, and the people of Chedi, Kasi and Karusha, he will maintain the prosperity of the Kurus. (Surfeited with libations at the sacrifice of king Swetaketu), Agni will derive great gratification from the fat of all creatures dwelling in the Khandava woods (to be burnt down) by the might of this one's arms. This mighty hero, vanquishing all the effeminate monarchs of the earth, will, with his brothers perform three great sacrifices. In prowess, O Kunti, he will be even as Jamadagnya or Vishnu. The foremost of all men endued with prowess, he will achieve great fame. He will gratify in battle (by his heroism) Sankara, the god of gods (Mahadeva), and will receive from him the great weapon named Pasupata. This thy son of mighty arms will also slay, at the command of Indra, those Daityas called the Nivatakavachas who are the enemies of the gods. He will also acquire all kinds of celestial weapons, and this bull among men will also retrieve the fortunes of his race.'
During the birth of Arjuna, it was said he would extremely great things, always invincible in battle, defeat powerful enemies whom even gods cannot achieve and collect all divine weapons which prove that Arjuna was divine and even divine gods, apsaras and nagas chanted his praise when he was born.
The various tribes of celestials assembled together, began to offer their respectful adorations to the son of Pritha. The sons of Kadru (Nagas), the son of Vinata, the Gandharvas, the lords of the creation, and the seven great Rishis, viz., Bharadwaja, Kasyapa, Gautama, Viswamitra, Jamadagni, Vasishtha, and the illustrious Atri who illumined the world of old when the Sun was lost, all came there. And Marichi, Angiras, Pulastya, Pulaha, Kratu, Daksha the lord of creation, the Gandharvas, and Apsaras, came there also. The various tribes of Apsaras, decked with celestial garlands and every ornament, and attired in fine robes, came there and danced in joy, chanting the praises of Vibhatsu (Arjuna). All around, the great Rishis began to utter propitiatory formulas. And Tumvuru accompanied by the Gandharvas began to sing in charming notes. And Bhimasena and Ugrasena, Urnayus and Anagha. Gopati and Dhritarashtra and Suryavarchas the eighth, Yugapa and Trinapa, Karshni, Nandi, and Chitraratha, Salisirah the thirteenth, Parjanya the fourteenth, Kali the fifteenth, and Narada the sixteenth in this list, Vrihatta, Vrihaka, Karala of great soul, Brahmacharin, Vahuguna, Suvarna of great fame, Viswavasu, Bhumanyu, Suchandra, Sam and the celebrated tribes of Haha and Huhu gifted with wonderful melody of voice,--these celestial Gandharvas, O king, all went there. Many illustrious Apsaras also of large eyes, decked with every ornament came there to dance and sing. And Anuchana and Anavadya, Gunamukhya and Gunavara, Adrika and Soma, Misrakesi and Alambusha, Marichi and Suchika, Vidyutparna and Tilottama and Ambika, Lakshmana, Kshema Devi, Rambha, Manorama, Asita, Suvahu, Supriya, Suvapuh, Pundarika, Sugandha, Surasa, Pramathini, Kamya and Saradwati, all danced there together. And Menaka, Sahajanya, Karnika, Punjikasthala, Ritusthala, Ghritachi, Viswachi, Purvachiti, the celebrated Umlocha, Pramlocha the tenth and Urvasi the eleventh,--these large-eyed dancing girls of heaven,--came there and sang in chorus. And Dharti and Aryaman and Mitra and Varuna, Bhaga and Indra, Vivaswat, Pushan, Tvastri and Parjanya or Vishnu, these twelve Adityas came there to glorify Pandu's son. And, O king, Mrigavyadha, Sarpa, the celebrated Niriti, Ajaikapada, Ahivradhna, Pinakin, Dahana, Iswara, Kapalin, Sthanu and the illustrious Bhaga--these eleven Rudras,--also came there. And the twin Aswins, the eight Vasus, the mighty Maruts, the Viswedevas, and the Sadhyas, also came there. And Karkotaka, Vasuki, Kachchhapa, Kunda and the great Naga Takshaka,--these mighty and wrathful snakes possessed of high ascetic merit also came there. And Tarkshya, Arishtanemi, Garuda, Asitadvaja,--these and many other Nagas, came there, so also Aruna and Aruni of Vinata's race also came there. And only great Rishis crowned with ascetic success and not others saw those celestials and other beings seated in their cars or waiting on the mountain peaks. Those best of Munis beholding that wonderful sight, became amazed, and their love and affection for the children of Pandu was in >consequence enhanced.
There were divine voices during births of Nakula and Sahadeva also.
Madri thought of the twin Aswins, who coming unto her with speed begat upon her two sons that were twins named Nakula and Sahadeva, unrivalled on earth for personal beauty. And as soon as they were born, an incorporeal voice said, 'In energy and beauty these twins shall transcend even the twin Aswins themselves.'
Pandavas were virtuous, honest, followed the path of virtue, had divine births for being the sons of gods themselves being born on auspicious days, having divine voices tell their glory and that they would be virtuous.
Description of Duryodhana:
The evil-minded and wicked king Duryodhana, the destroyer of the fair fame of the Kurus, was born of a portion of Kali on earth. He it was who caused all creatures to be slain and the earth to be wasted; and he it was who fanned the flame of hostility that ultimately consumed all.
Mahabharat describes that Duryodhana was the one who caused a lot of creatures to be slain whereas the Pandavas first offered peace despite the fact that Kauravas tried to kill them by various means even in childhood.
Yudhisthira was patient and caring for those that are his companions such as the dog (Yama in disguise) during the journey of Himalaya Mountains.
During Mahaprasthanika Parva when Yudhisthira is about to enter the chariot for heaven, Indra tells to abandon the dog.
"Indra said, ‘There is no place in Heaven for persons with dogs. Besides, the (deities called) Krodhavasas take away all the merits of such persons. Reflecting on this, act, O king Yudhishthira the just. Do thou abandon this dog. There is no cruelty in this.’
"Yudhishthira said, ‘It has been said that the abandonment of one that is devoted is infinitely sinful. It is equal to the sin that one incurs by slaying a Brahmana. Hence, O great Indra, I shall not abandon this dog today from desire of my happiness. Even this is my vow steadily pursued, that I never give up a person that is terrified, nor one that is devoted to me, nor one that seeks my protection, saying that he is destitute, nor one that is afflicted, nor one that has come to me, nor one that is weak in protecting oneself, nor one that is solicitous of life. I shall never give up such a one till my own life is at an end.
It shows that Yudhisthira cares for his companion.
Even Yama praises him as follows,
"Dharma said: ‘Thou art well born, O king of kings, and possessed of the intelligence and the good conduct of Pandu. Thou hast compassion for all creatures, O Bharata, of which this is a bright example. Formerly, O son, thou wert once examined by me in the woods of Dwaita, where thy brothers of great prowess met with (an appearance of) death. Disregarding both thy brothers Bhima and Arjuna, thou didst wish for the revival of Nakula from thy desire of doing good to thy (step-) mother. On the present occasion, thinking the dog to be devoted to thee, thou hast renounced the very car of the celestials instead of renouncing him. Hence. O king, there is no one in Heaven that is equal to thee. Hence, O Bharata, regions of inexhaustible felicity are thine. Thou hast won them, O chief of the Bharatas, and thine is a celestial and high goal.’"
Yama himself praises Yudhisthira to be virtuous. Even in heaven, Yama praises Yudhisthira.
‘O king, I am greatly pleased, O thou of great wisdom, with thee, O son, by thy devotion to me, by thy truthfulness of speech, and forgiveness, and self-restraint. This, indeed, is the third test, O king, to which I put thee. Thou art incapable, O son of Pritha, of being swerved from thy nature or reason. Before this, I had examined thee in the Dwaita woods by my questions, when thou hadst come to that lake for recovering a couple of fire sticks. Thou stoodst it well. Assuming the shape of a dog, I examined thee once more, O son, when thy brothers with Draupadi had fallen down. This has been thy third test; thou hast expressed thy wish to stay at Hell for the sake of thy brothers. Thou hast become cleansed, O highly blessed one. Purified of sin, be thou happy.
Yudhisthira hence deserved to be the king of Hastinapura and was the rightful heir to the Kuru throne and also, Yudhisthira and his four brothers were extremely powerful, sons of gods themselves born on auspicious days and divine voices sang their glories when Yudhisthira and his brothers were born. Yudhisthira on account of his honesty, frankness, and virtuous behavior was the rightful heir to the throne of Hastinapur.