My top choice will always be V&A. It's the only place guaranteed to be romantic. And the food is in a league of its own.
Next up I would pick one of the Swan/Dolphin restaurants as they have a tendency to have a more adult clientele and aren't as packed since they don't offer the dining plan. My personal favorite is blue zoo but Il Mulino and Shulas are equally good, depending on the type of food you are looking for. As long as you are not there on a night with conventioneers, these places are usually very quiet.
Bistro de Paris upstairs in Epcot can be romantic, we've always had great meals there, but there have been recent reports of disruptive children dining here (they don't have a child's menu so it used to be a great adult spot).
Portabello Yacht Club at DTD is very nice also. I don't think the food is as good as Il Mulino at the Swan (both are Italian) but it's in a pretty setting.
After these, I'd probably pick Citricos first out of Disney's restaurants. Next would be Jiko then Artist Point.
If you are wanting to spend signficantly less, I'd recommend Turf Club at SSR. It's a bit of a hidden gem and we've enjoyed quiet meals here too.