There are many different stories on Ramayana. Which is the original ? And, why was it manipulated by so many great writers ?
The original and authentic version is Ramayana written by Sage Valmiki. It is considered as authentic version because he has a boon from Brahma that he would know the feelings and thoughts of the Ramayana character on his wish.
रहस्यम् च प्रकाशम् च यद् वृत्तम् तस्य धीमतः || 1-2-33
रामस्य सह सौमित्रे राक्षसानाम् च सर्वशः |
वैदेह्याः च एव यद् वृत्तम् प्रकाशम् यदि वा रहः || 1-2-34
तत् च अपि अविदितम् सर्वम् विदितम् ते भविष्यति |
The adventures of valorous Rama along with Lakshmana, and the misadventures of demons, known or unknown in every detail, and even the plight of Vaidehi which is either revealed or un-revealed so far, and whatever legend that has happened, all that will also be known to you, even if it were to be unknown, as yet... [1-2-33b-35a]
न ते वाक् अनृता काव्ये काचित् अत्र भविष्यति || 1-2-35
कुरु राम कथाम् पुण्याम् श्लोक बद्धाम् मनोरमाम् |
You shall versify the heart pleasing and merit-yielding legend of Rama, and not a single word of yours will be unfounded in this epic... [1-2-35b-36a]
It is the unparalleled version. He appreciates Ravana equally to Rama or Hanuman and doesn't show any difference as Ravana is a negative character.
For example, in the Sundara Kanda,
स तथाप्युग्रतेजाः सन्निर्धूतस्तस्य तेजसा |
पत्रगुह्यान्तरे सक्तो हनुमान् संवृतोऽभवत् || ५-१८-३१
Thus Hanuma although of terrible energy was overshadowed by Ravana's brilliance and became rooted behind a group of leaves and was concealed.
Here, Valmiki shows the brilliance (tejassu) of Ravana. It clearly says that Hanuma was overshadowed by Ravana's energy. This is just an example of his impartiality. If Valmiki wanted to defame Ravana, there wouldn't be such verse. There are many instances where Ravana is praised as great.
Later many great writers got inspired by this Ramayana and wrote their own Ramayanas. Some didn't like this impartiality. Some translated the book into their mother tongues and some modified and some recreated it by adding (eg: lakshmana rekha, ) and modifying (eg: abduction of Sita) some incidents.
But the base story remains the same. But Valmiki Ramayana has a special essence in it.