Very difficult question because I love a LOT of the WDW-music and I hear it a lot. I have dozens or more like a hundred or more of MP3s including complete BGM loops. My all time favorites are the Main song of the Tiki Room (the original Tiki Room, not the present abomination) and the HM-main theme. At the moment I would vote for the Soarin'-theme however.
BTW I always found it quite disappointing that another classic, the Jungle Cruise NEVER had a theme of it's own. I think every major ride needs an accompanied tune because the music is an important part of being able to relive the experience of the ride at home. We can do this with PotC, HM etc. but if you are at home there is no way of bringing the JC into your home.
And does anyone know where I get the new Space Mountain theme?