The best (also one of the most expensive) restaurant is Walts. It's above some of the shops on Main Street U.S.A. They serve a variety of dishes but is also aimed at, I suppose, a more upper-class experience. Each room is themed to each land in the park and the walls are decorated by art that inspired the look of the park and concept art for the park and rides. You can't get more original than that!
The Lucky Nugget in Frontierland is a nice place to eat. It's a table service restaurant that only sells fast food items (quality can differ). From time to time a live band plays on the stage to entertain you while you eat.
Agrabah Cafe in Adventureland is a buffet service restaurant that tends to serve a decent mix of food. Buffet choices might not be as varied as you're used to at the American parks. The interior is amazing and creates a fantastic atmosphere.
Plaza Gardens (also on Main Street) is another buffet service restaurant but provides a more generic offering of food.
In terms of fast food places you have Toad Hall (Fantasyland), Pizzeria Belle Notte (Fantasyland), Cowboy Cookout (Frontierland) and Colonel Haithi's Pizza Outpost (Adventureland). They're the best and most unique I think. Cowboy Cookout has changed recently though so I'm not sure what that's like anymore.
In the Studios food options aren't so varied. There's the buffet, Restaurant des Stars, which is ok and serves a similar menu to Plaza Gardens. The Studios mainly has fast food options which are average.
The Village also shouldn't be ruled out. King Ludwigs is one of the best restaurants in the village and the interior is fantastic. There's also an Earl of Sandwich (I'm a huge fan), Annettes (50's American diner), Rainforest Cafe and Cafe Mickey (if character dining is your thing).
If you have time in the evening and the village isn't offering something to your liking then you can access the restaurants in the hotels which all back on to the lake at the end of the Village.
For menus (I think most of these have changed since taken) to give you a good idea of what's on offer it's best to go to
DLRP Magic
Any more questions, I'm more than happy to help.
