I have three from the theme parks - Wishes, Spectro Magic, and the one for Main Street, which on my WDW cd is three seperate songs in a medley; Walking Down Main Street, the Trolley Song, and I can't remember what the first one's called, but they always put me in a Disney mood.
Then I have certain ones from the movies that I always think of - A Dream Is A Wish Your Heart Makes, which I think really represents Disney, and agreeing with the rest of the posters who said it, When You Wish Upon a Start. Also, I really like A Whole New World, Can You Feel the Love Tonight, and Part of Your World.
I hope it's all right that I picked so many, but I just can't pick one.