MK -- Tie between Cosmic Ray's (three separate service counters + three completely different menus = something for everyone) and Columbia Harbour House (just for it being a completely different menu than everyplace else).
Epcot -- I didn't eat counter service last time I was there, but my mouth was watering at the prospect of Tangierine Cafe.
AK -- Tusker House and Flame Tree menus looked great, but due to geography as well as disagreeable in-laws we ate lunch at Restaurantosaurus.
AVOID RESTAURANTOSAURUS AT ALL COSTS (other than breakfast). They have McDonald's fries and McNuggets, but they have their own brand of dry and overcooked 1/4 pound hamburgers, and there is no child-size burger and there is no kids meal (with a toy, etc.) for the burgers. Lame menu, poorly done, but on the plus side they have a pretty awesome fixins bar and the restaurant itself is pretty large inside.
MGM -- Backlot Express. For months, all my five year old could talk about was Toy Story Pizza Planet. I guess it was the only place in all of WDW that I DIDN'T have the menu memorized (

to
www.allearsnet.com for the online menus), because I was really disappointed in the very limited menu. My wife ran around to Backlot Express and got us both great, interesting sandwiches.