Magic Kingdom - Cosmic Ray's. This is the only park where I just end up getting burger and fries and loving it. Plus they have the huge toppings bar and you can get cheese for your fries.
Epcot - Tangerine Cafe by far. It's going to be the best value for your credits, as it's the most expensive counter service place. The shwarma platters are great. I also really like Lotus Blossom over in China, they have great fried rice.
MGM - Studio Catering Company reopened just last year with a new menu. It's Mediterranian, with wraps and greek salads and things like that. The greek salad and the chicken ceasar wrap are really good. Everyone on here is complaining about MGM counter service, and if it weren't for this place, I probably would be, too. It's one of MGM's hidden treasures, it's way in the back by Lights Motor Action and the Honey playground.
Animal Kingdom - Anywhere but Restaurantasaurus (which is just a McDs). I'm not a fan of the type of bbq over at Flame Tree, but I've heard nothing but good things about it. The sandwiches at Pizzafari look great, but I've never had them. Tusker House is the only one I've eaten at and their rotisserie chicken is really good.
You can also use them at the counter service places at the hotels. I'm a cast member, but I recently got a bunch of counter service meal vouchers and was using them all over the place. Pop Century has really good meals (like the fried chicken tv dinners), as well as Port Orleans Riverside. The Contemporary has a great chicken tender sandwich.