I say Animal Kingdom, 'cause its the only park that hasn't become totally screwed up at WDW.
It's original and well-themed.
ETA: I realize some people will take that the wrong way and I just want to say- I love WDW, I love Disney parks. I, however, realize that Disney and WDI can achieve very high quality, and it's been missing the mark lately. It doesn't affect my enjoyment of the parks at all, but it does sadden me at home, to read about it.