IMO, Crystal Palace and Chef Mickey's are best...but I never ate in the castle, so that might be better for all I know.
I do Crystal Palace on MK days, but I schedule it late so that I can run around like a lunatic riding Space, Splash and Jungle Cruise (or Dumbo, Pooh and Peter Pan, with a young one) first. The restaurant is so pretty. I think if it weren't so loud and busy, it would be my favorite restaurant for atmosphere. And that Puffed French Toast on the kids table?! Yum! (Not French toast at all, more like tasty fried dough, but so good.)
Chef Mickey's is at the Contemporary and would suck more time out of a vacation than you might want to waste. It also has a more hectic feel to it, even though it really isn't more hectic. But it has Mickey.