As we know there were many people who could have killed Ravana, like Lord Hanuman and Jambavan. But they didn't kill Ravana. Only Lord Rama did. Why?
One important reason goes far back to even before Ravana was born. Ravan was actually one of the two gatekeepers at Lord Vishnu's abode Vaikuntha. Once, they stopped the four Sanat kumaras from entering into Vaikuntha and they got cursed. So they took take birth in demonic families as the Lord had desired:
The Lord replied: O brāhmaṇas, know that the punishment you inflicted on them was originally ordained by Me, and therefore they will fall to a birth in a demoniac family. But they will be firmly united with Me in thought through mental concentration intensified by anger, and they will return to My presence shortly. [SB - 3.16.26]
They later on took birth as Hiranakshya, Hirankashyapu in Satya Yuga, Ravana, Kumbhakrana in Treta Yuga, and Kamsa, Sisupala in Dwapara Yuga. So for their deliverance, Vishnu in His avatars had to kill them. So only Lord Rama Killed Ravana.
Jambvan can't kill Ravana, because Ravana had a boon that only a Monkey or Man ( Narayan in man form) can kill him. Jambvan was in form of bear.
When Ravana met first time with Lord Hanuman, he told him that He is Amar ( a person who can't die ). Listening to this, Lord Hamuman reminded him his boon that he was blessed to be only killed by a Monkey or Narayan ( i.e. Lord Vishnu ).
Hanuman told him that he too can kill Ravana at that very moment but he wanted Lord Rama to get credit of killing Ravana, and that's why he didn't kill him.
"any people who could have killed Ravana, like Lord Hanuman and Jambavan. But they didn't kill Ravana."
Not sure about Jambavana, but Hanuman had the ability of killing RAvana along with his whole army. However, he didn't kill him, because he wanted to give the credit (glory) to the lord RAma as stated in [Tirth yatra parva]:
Then with his down standing erect, the noble-minded Bhima, joining his hands, replied unto Hanuman saying (there), 'O lord, by me have been beheld the vast dimensions of thy body ... Depending on the strength of thy arms, thou wert capable of instantly destroying Lanka, with its warriors, and horses, elephants and chariots. Surely, O son of the wind-god, there is nothing that is incapable of being achieved by thee; and in fight, Ravana together with his followers was no match for thee single-handed."
Hanuman, the chief of monkeys, answered in affectionate words uttered in solemn accents. "O mighty-armed one, O Bharata, it is even as thou sayest. O Bhimasena, that worst of Rakshasas was no match for me. But if I had slain Ravana--that thorn of the worlds--the glory of Raghu's son would have been obscured;--and for this it is that I left him alone. By slaying that lord of the Rakshasas together with his followers, and bringing back Sita unto his own city, that hero hath established his fame among men.
As pointed out above, ravana could only be killed by a vaanar(monkey) or a manushya(human).
Secondly, when hanuman met ravana for the first time, he went as a doot(ambasador) and couldn't fight with ravana.
Also, you always fight with a person of your stature, therefore, ravana(leader of rakshasa) fought with lord Rama.