AK: Festival of the Lion King and EE are my two favorite attractions in ALL the parks...but I think I'd give the slight advantage to FotLK!
MK: Space Mountain
Epcot: Living with the Land
DHS: Tower of Terror
MK - Haunted Mansion
AK - Safari
DHS - Great Movie Ride
EP - This is a tough one because my favorite things to do there are not attractions. I love to nosh, shop and drink when we're there, but I guess I'll say Maelstrom because it's so unique.