Hmm... best bang for the buck huh? I'd say that most of the buffet-style places are pretty good buys. Trail's End is a family favorite. The character buffets such as Crystal Palace tend to be a bit pricier, but the "show" is usually better there too. There are exceptions to this however, in places such as the Whispering Canyon Cafe, one of my absolute favorite places to eat. The atmosphere is great, the "show" is fantastic, and if you get their signature Canyon Skillet, it's all you can eat, and for a moderate price. I love it.
Another place with fantastic atmosphere that I love is the 50s Prime Time Cafe, but I wouldn't necessarily say it's the best deal when it comes to quality and quantity of food for the dollar spent. Also, someone else already mentioned La Cantina de San Angel in Mexico as a good sized meal for a fair price, and I'd have to agree with them, but only if you actually enjoy Mexican. There is very little variety at most Word Showcase locations, provided their purpose is to showcase foods from their native lands. While I enjoy the place, my girlfriend can't stand Mexican food, and so it's not even an option when we're there together.
As for the debate about best burgers and their fixin's, I 'd have to agree with BryanFykes about the burgers in the parks all being the same. However, I don't recall seeing the sauteed mushrooms on the toppings bars (although I could be wrong) any place other than Pecos Bills. But I wouldn't go so far as to say all burgers are the same OUTSIDE the parks... I've got a new favorite place that I just tried for the first time a few weeks ago. It was already mentioned here earlier in the thread, but I'll throw it out here again - Beaches and Cream, in front of the Yacht & Beach Club Resorts. It's an old fashioned soda shoppe, where you can pull up a stool at the counter and watch them make your burgers, shakes, sodas, and ice creams right there. It's really good, and the prices are great. The burger I got there was far superior to any I'd ever gotten in the parks. I think my girlfriend and I averaged about $12 a piece for our dinners and drinks. And the ice creams are all pretty huge... especially if you order the Kitchen Sink. I don't recommend that one for any parties smaller than 4 however... It's a big dessert!