I agree, Ravjay12, about "When You Wish Upon a Star." There are times I make it a point to walk through the castle on my way out, just so that song can be fresh in my mind when I leave. Hopefully it can be stuck in my mind for the trip home.
I'm listing my top five because it's impossible to pick just one.
1. Grim Grinning Ghosts- Haunted Mansion
2. The soundtrack from Roger Rabbit's Car-Toon Spin
3. How Do You Do- Splash Mountain
4. Space Mountain soundtrack
5. California Screamin' soundtrack
And I guess one honorable mention would be "There's a Great Big Beautiful Tomorrow", although I'm not sure if I like it or hate it because it has a tendency to get stuck in my head for hours and hours on end. It's worse than Small World, I tell ya. XD
At all the Disney parks I truly appreciate the background (at least back in the day when it was mostly original music).
"The Swiss Polka" at the threehouses.
"The Monorail Song" and "Nation on Wheels" in Tomorrowland
Instrumentals of "Whale of a Tail" adjacent to the Submarine lagoons and inside Columbia Harbor House
"The Sons of Liberty" playing in Liberty Square and at the American Pavilion
"Natures Wonderland" soundtrack and "Old Betsy" playing in the queue of Thunder
"The Ballad of Pecos Bill" playing in Frontierland
I has bothered me lately that Disney has started to remove many of these great original music pieces in the background and replacing them with non-Disney and sometimes contemporary music, that distracts from the show element.
For instance John Philip Sousa (while patriotic) doesn't necessary belong in the "Colonial" show scenes of Liberty Square and the American Pavilion. And La Bomba doesn't belong in the background of Frontierland. Music from Lord of the Rings definitely doesn't belong at EPCOT.
Disney has had the fortune of employing some amazing song writers throughout its history. The first 50 years of the company produced hundreds (if not thousands) of original songs and compositions. Why has that waned over the past few decades? Is it just the nature of the entertainment industry where artists as no longer "under contract" to the studios?