Hm, it's tough. There are several vastly different reasons that I enjoy each park.
I think I'd have to go with...
1.01) MK - Rides, Atmoshphere
1.02) Epcot - Rides, Food
1.03) AK - Atmosphere, Rides
4) DHS - Atmosphere at night only, Rides
I also approach each park different. With MK, it's more of a general "go with the flow" type of thing, whereas DHS has a specific setup due to other things I want to do (mostly to do with my camera). I do make sure to hit each park at least twice during the day and once or twice at night during each trip (yes, I use hoppers a lot
) since the atmospheres in each park change considerably between day and night.