AK: Satu'li canteen is hands down my favorite QS in any park (and is at the top of my favorite restaurant on property, period). It has great customizable variety and healthy options (including vegetarian/vegan). And, I love both deserts.
MK: Harbor House has good variety and healthy options, and "lighter" fare. As others have said; go upstairs and look around for some neat places to sit.
EPCOT: if you are toward the front of the park; Sunshine Seasons. Again, great variety and unusual fare, lots of seating. If in WS, then Tangierine Cafe in Morocco, their food is very filling and good, but doesn't weigh you down. If you want a more starchy variety and the best deserts, go to Les Halles in France.
HS: This is the hardest park. We go to Backlot out of sheer tradition, but I've heard good reviews about Woody's for certain food.
Those were all for lunch/supper. If you want a quick bit for breakfast, we love Sleepy Hollow in MK (esp. for a fruit and nutella waffle).