[quote="Noahcs"]1.
Busa (probably my real favorite)
[/quote]
Technically, Busa has lyrics, and therefore is not an instrumental.
Out of the list, I would say that I love "Under The Stars" the best, but I'd hate to say it is the best. Because honestly, the entire musical score is amazing, and you really have to watch the movie (and try and ignore words/songs) to fully appreciate it.
Also, to add to TheLionPrince's list:
Battle of Pride Rock This is the instrumental that plays when Simba returns home, and ends when the movie ends. There is heavy overlap in the end with "King of Pride Rock", but there are parts that are not herd in any other material except for the movie, such as the climax when Simba and Scar fight. Excellent 30 seconds of music, that climax is.