A trip to WDW without riding Haunted Mansion at least 3 times is a travesty. It's really the only attraction I feel I must do every visit. And maybe Pirates.
MK - Pirates, Tomorrowland Transit Authority, Peter Pan
AK - Everest, Nemo Show
Epcot - World Showcase as a whole, Soarin
DHS - Star Tours, Tower of Terror, Fantasmic
Believe it or not, Living with the Land is my favorite ride at Epcot. Splash is my favorite at MK. I enjoy most everything else as well, but those two really lift my spirits and give me that "ahhh-I'm-here-feeling!"