I'd recommend....anywhere you want to eat!
San Angel Inn can be very romantic, and we love it even if many here don't. Le Cellier might be nice.
Sci Fi Dine could be romantic as well in a fun kind of way. Time to snuggle in the "car" watching bad sci fi trailers while you wait for food...romantic, right?
I've always liked the Brown Derby.
It really all comes down to what you're looking for. A quiet, adult meal? Stick to the signature restaurants. Don't mind some noise and craziness? Places like Ohana can be good.
I'm personally excited about having dinner at the castle during my upcoming trip. I haven't been since dinner became a character meal, so I don't know if it will be romantic, but ...it's the castle.