Cosmic Rays at MK. Fun place, lots of choices. And if you are on the dining plan, you can get the chicken and ribs as a choice!
Animal kingdom: Flame Tree BBQ fantastic chicken. When ex and i went in 2001, it was our favourite counter place meal of the trip, and the best value (one of the few things we ever agreed on ROFL)
as for Epcot, I imagine that the counter service area in the land would be the best? However, that said, I plan on making a stop at Mexico on our next trip. :O)
And for MGM I have only had food at 50's prime time cafe.