There are tons of good (and bad) restaurants at Disney, but knowing your specific likes and dislikes will help with our recommendations. Also, will you have a car or be relying on Disney transportation?
For Table Service Restaurants:
In Magic Kingdom I personally don't think any of the restaurants are worth using a credit on. If I had to I would probably pick Crystal Palace (keep in mind it's a character meal though) or Tony's Town Square.
Epcot...so many to choose! I would pick Le Cellier for lunch (only on table service credit), Either of the Japanese restaurants for dinner (if you're not a big fan of sushi than Teppan Edo would be a better fit).
Hollywood Studios - again wouldn't waste a TS credit here.
AK-wouldn't use a TS credit here either.
I would recommend eating at some of the resort restaurants. You could always leave MK early and eat at Kona (in the Polynesian) or when leaving Animal Kingdom I would go over to the Animal Kingdom lodge and eat at either Boma or Sanaa.
For Downtown Disney I would probably suggest Wolfgang Puck for a TS credit. Starting in 2012 more DTD (downtown disney) restaurants have been added so you might want to also check out their menus.
For counter service restaurants here are my faves:
MK-Cosmic Ray's - great variety and best bang for your credit
EP-Sunshine Seasons -same as above
DHS-Backlot Express - best for what the park has to offer (which is not much)
AK-Flame Tree - same as DHS
DTD-Wolfgang Puck Express (I would eat here everyday if I could

), Earl of Sandwich (not as good use of a CS credit but definitely good sandwiches)
If you used some TS credits at lunch I would use some CS credits for Dinner (like Wolfgang Puck Express). If you do a google search or even a search here on the boards I'm sure you'll read all the rave reviews of WPE (plus all
the pics

)
What resort are you staying at? This will help on recommendations too!