This question is always a tough one to answer. Even though all four parks offer so much, and each are just over-flowing with Disney Magic, for me the two parks that provide the most magic would be the Magic Kingdom and Epcot.
Both of these park could easily claim my vote for which is the "best", and having to decide between them is almost akin to having to choose which of my kids I like the best. However, this poll isn't asking me which are my favorite parks, rather which ONE is my favorite.
I went ahead and voted for Epcot. If I were voting which is my family's favorite park, or which park I like to spend the most time in with my family, then I would have gone with the Magic Kingdom. But, since I'm being selfish, I'll go with the one that personally offers me the most magic, and that would be Epcot.
If I only had a chance to visit one park (and I was alone, or with my wife), I would go to Epcot. If my kids were present, then I would go to the MK.
Tough question.