Well, it depends upon your criteria for ambiance and food. There will be tons of varying opinions, so why not toss mine in?
Victoria & Albert's - Far and away, the best restaurant on property. Only cons are very expensive and hard to snag reservation. Assuming that's a non-player. . .
AMBIANCE
Jiko - Good food, great service, lovely and quiet setting, beautiful resort, nice wine list
Il Mulino - Not quite as lovely as Jiko, but they have a great menu (they offer half plates of pasta), a wonderful wine list, and our service has always been good
BiCE - Though located at the Portofino Resort over at US, it's a tremendous restaurant with a topnotch menu, superb service, great wine list, and lovely views
CONVIVIAL
California Grill - Generally very good food and service, but it's noisy. Even if there aren't any kids running around, all the glass and the kitchen in the middle of the floor echo voices, clattering, and the like. The plus side is if you time it right (such as Ken's suggestion above), then you can see the fireworks from your table. Even if you don't, you can come back after dinner to watch them.
Enzo's Hideaway - Tucked away in Disney Springs, it's reminiscent of a speak easy, cellar restaurant. We found it nicely done with a touch of whimsy thrown in. Our servers were excellent, and the food was a hit, too. There are booths you should be able to grab if you ask nicely.
Sanaa - One of the best "bang for your buck" places at Disney. Food is very good; service is friendly; drink/wine list is solid; and the bread service is divine. Not upscale, but it's well-themed.
Morimoto - Very unique setting which can be a little loud (not CA Grill loud, though) but with generally good food, especially the available-only-at-dinner buri bop.
Kimonos - If you like sushi. If you don't, never mind.