My personal feelings are as such:
1) Epcot. Its definately seen the most innovative things added to it out of all the parks over the past 10 years or so. i.e. Mission: Space, Test Track, Turtle Talk with Crush, the brand new living seas update, ice station cool (club cool now), Soarin'/The Land Rehab, etc...
Plus: The World Showcase (amazing food and an amazing experience), and then finally my fav...Spaceship Earth. so that gets my vote.
but in less detail...
2)MGM/Disney Studios
3) Magic Kingdom and Animal Kingdom are basically tied. but i am POSITIVE that shortly (within the next 10 years) Animal Kingdom will jump far ahead. only because, along with Expedition Everest it is bound to see many more additions like Epcot has seen recently.
So thats my take on it. oh, and Magic Kingdom has it definate +'s to Disneyland...but Disneyland also has its special things as well. Its different enough to check out and appreciate. Im hoping Magic Kingdom will soon see some improvements liek Disneyland has gotten.