It honestly depends on the person... If you like thrill rides, probably DHS or MK are the best choices... but if you're like me and aren't a big fan of the thrill rides, AK and EPCOT are a lot of fun to just walk around and see stuff (and eat!) I think MK probably has the biggest variety of rides, but a lot of them tend to be geared towards younger children... Here's a list of what I've seen to be the most popular rides in each park:
MK - The mountains (Space, Splash, Big Thunder), Dumbo, Haunted Mansion
EPCOT - Soarin', Mission: Space, Test Track
DHS - Rockin' Roller Coaster, Tower of Terror, Toy Story Mania
AK - Expedition Everest, Kali River Rapids, Kilamanjaro Safari
Rides in green font are what I would consider "thrill rides" (but it's completely my opinion.. )