At first, I thought it was definitely MK, I mean, the castle, the magic you feel while you're there, and that's not even half of it! I think it's a really close tie with EPCOT.
EPCOT- I think I'd choose this one because it's got Test Track, Soarin, and the beautiful World Showcase. There's probably a few other things I missed, but that's pretty much what I love.
MK- It's got that beautiful castle, most of the rides that I love, the sensation of just BEING THERE, the different lands, and it just feels so magical. Also, I really wanna try HM. And, I've got so many memories of being there when I was little (mostly about crying on almost every single ride, but who cares?) and it is ALWAYS the very first park we visit whenever we go.
But, I guess if I had to choose one, it has to be MK. In reality, I would buy a park hopper, but I guess that isn't an option with this question.