I would have to go with EPCOT. I like it because it has future stuff and all the countries. But the Animal Kingdom is not far behind because I like the look of the park and the animals. Disney Studios is also cool because I like R'nR and ToT. MK. is OK not my fav.
I was going to say Epcot, but then I was thinking Magic Kingdom, although I had a great time in MGM and AK, too... I suspect it's the same way I feel about holidays...whichever one I'm celebrating is my favorite!