MK:
Cosmic Rays, hands down. There are plenty of good snacks at the MK, but Cosmic Rays is by far the best place to eat in the park. It has the best selection of food (it's got a darned good chicken caesar salad), Tomorrowland music, Sunny Eclipse, and great views of the Hub from the outside seating area.
The Plaza Pavilion is also a good choice. They have decent pizza, it always smells good there, and you get excellent views of the Hub and Tomorrowland.
EP:
Electric Umbrella - Standard fare, but it's a fun place to eat
Sunshine Season Food Fair - A nice selection and a large sitting area. Nice ambience too
Yakitori House - I love Yakitori, so this is a favorite of mine.
MGM:
Backlot Express - Standard fare, but the atmosphere is fun.
AK:
Pizzafari - They have the best sandwich on all WDW property (a hot deli sandwich)