Age plays a huge decision in this one. For us it is the MK. The reason is simple: there is more to do there than the other parks combined. This is the sad truth. You have the following
Rides (20+): Aladdin, Jungle Cruise, PotC, Splash Mtn, BTMRR, HM, IASW, Peter Pan, Carousel (sp?), Pooh, 7DMT, Tea Cups, Barnstormer, Ariel's ride, Speedway, Space Mtn, Astro Orbiter, People Mover, Buzz, Stitch (does it count?)
Attractions (12+): Swiss Family Tree, Country Bears, Philharmagic, Storytime with Belle, Princess M&G, Anna/Elsa M&G, Ariel M&G, Tomorrowland Dance Party, Monster Inc Laugh Floor, Fairy M&G, Mickey M&G, all other M&G's, everything else I forget from memory
Shows (10+): Castle stage shows, parades, MiSi (ending?), fireworks, moving trash can, all other misc things that go on (example Damper Dans which aren't there any more).
Simply put, there is so much that can be done there. Epcot is the only other park on par with enough to do and experience. DHS and AK can easily be done in a half day each. Time DHS right and you can do the park in 3 hours.