I'd have to go with Magic Kingdom first because it's the original and nothing compares to walking down Main Street for the first time on a trip. Nothing.
But after that it's a tie between Epcot and MGM because I love World Showcase, especially Italy because I want to go there so bad and Venice would be my first stop, but MGM has my all-time favorite ride ever, Rock N' Roller Coaster. So I'll say that they're tied for second.
Poor, poor Animal Kingdom. We usually spend about an hour or so there, just for the Safari and Dinosaur.