Many will say Victoria and Albert's in the Grand Floridian. I have never been, but it is quite upscale and I'm sure very nice and romantic.
My DH and I ate at California Grill in the Contemporary on our last visit and really enjoyed it. It was very nice, and I'd say more upscale and less "crowded" than say the restaurants in EPCOT. We dresssed up, and there is a resort casual dress code they enforce. We will return for my birthday dinner next month. The view of the castle is amazing.
We will also be trying Citricos in the Grand Floridian for ther first time next month.
By the way, both of these restaurants are about half the price of Victoria and Albert's, just as a point of reference.
Hope that helps.