It all depends on what you like.
I'm a 30 year old tall kid with a big allowance and no curfew who's also more than a bit of a geek (aerospace engineer). Epcot is my favorite park, The Future World part of it. I could spend half a day in innoventions, easy.
My sister, on the other hand (three years older than me and more of the liberal arts bent) loves Animal Kingdom. She could ride the Safari all day.
My favorites at each park:
MK: Buzz Lightyear (just don't put my sister and I in the same car, too much sibling rivalry, although my BIL puts us all to shame when it comes to tallying points at the end) and Splash Mountain. Mom love Pirates of the Carribean and Jungle Cruise (but Mom likes puns and bad jokes).
Epcot: Innoventions and Living Seas. Everyone in the family likes Test Track. I'm looking forward to Mission Space (wasn't open last time I was there). Mom (an Occupational Therapist, so she goes in for the med stuff) won't ride Body Wars but could spend half the day in that pavilion. My 5 year old nephew loves Soarin'. We all like the 30 minute, air conditioned nap that Universe of Energy provides.
Futureworld: I don't have a favorite here, but Mom and Dad like these. Dad likes the Norway ride. Mom likes the China movie. I like the live singers at the American Pavilion, but other than that, I'm not a huge fan.
MGM: Rockin' Roller Coaster (The takeoff is amazing) and the Muppets pre-show. The Muppets show itself is okay, but the preshow and wandering around the "backstage" area is hilarious. But, I also grew up watching the Muppets every Saturday night.
Animal Kingdom: Safari, the Birds of prey show (I like things that fly) and Dinosaur (although I like Indiana Jones at
Disneyland better even though it's pretty much the same ride). Mom loves watching the tigers. My nephews like the dinosaur playground.
As for shows, my sister and her family like Illuminations (they do have the best fireworks). My older nephew and I like Fantasmic. Mom and Dad go in for Spectromagic. I enjoy watching the water parade and the MK fireworks from the Fort Wilderness beach. A nice, quiet time (although it can get chilly since we usually go for Thanksgiving).