I find it difficult to recommend Beaches and Cream when a very short walk away, you can go to the Fountain - which is a little cheaper all around and at least in my opinion, better quality burgers, fries, and shakes - plus a good selection of other sandwiches.
Generally, I consider good bang-for-the-buck to be tasty food served in big quantity - I feel like I got my money's worth if I can't finish the plate but it was very tasty. Art Smith's Homecomin' is great for that. House of Blues is always a favorite easy-eating spot. San Angel Inn always works because you can fill up on chips and salsa as well as a good size serving on your main.
I eat at many of the buffets, but rarely consider them to be bangs for the buck - only because the price is typically higher than a table service order - it is all you can eat, but I rarely end up eating enough to feel like I came out ahead. I still like the places - I just wouldn't put them on the better end of the bang-for-the-buck quotient.
Rainforest and T-Rex are decent for the money - typical American chain restaurants with big heaping servings and overflowing plates - the food quality is average but the amount for the price is always good.
**And just for the pure bargain hunters...probably some of the best pricing anywhere on Disney property would be Chip n Dale's at Saratoga Springs. Not table service - It's the cafe for the golf course, but anyone can eat there - just thought it was worth mentioning as it's the only place I can think of at Disney where you can get a sandwich, chips, and a drink for $13 or less!