It's awfully close, between MK and Epcot. But I think Epcot wins. I don't know why, but that is definitely where I spent the most time when I was in the College Program. And it's the first place I want to visit when I go back.
No fair - you didn't give the option of a tie! I love both MK and Epcot so much, for very different reasons, it's hard to compare them and choose one. I also love Disney Studios and even AK in it's own way, but MK and Epcot are tops. The tough part is choosing just one of those two!