There are four answers to this question:
Magic Kingdom - if you like parades (Share A Dream Come True daily and Spectromagic busy nights) - classic rides (Haunted Mansion, Pirates, Small World, Jungle Cruise) - the castle - classic fireworks (Fantasy In The Sky busy nights)
MGM - if you like shows (Little Mermaid, Beauty and Beast, Indy, Walt Disney film, Millionaire, Fantasmic) - thrills (Tower of Terror, Rock n Roller Coaster) - tours (animation, tram)
Epcot - if you love to eat (many, many countries) - educational peices (American Adventure, Canada and Italy films) - and a great ending of the day (Tapesty of Dreams and Reflections)
Animal Kingdom - if you like animals (safari ride, all the walk around areas) - if you like shows (Festival of the Lion King, Tarzan Rocks, Pocahontas, Birds) - if you like beauty (all the plants, the tree of life) - if you like a peaceful, carefree day --- Maybe the best parade according to several on the message boards (Mickey's Jammin' Jungle Parade).
All I would say are one day parks - not saying you can do everything in a day, but you can stay busy for the entire day.
With a question like this, you can't go wrong. There isn't a loser answer.