I suggest Paris. When I went there I understood why they call (old) Paris the romance capital of the world.
Only problem: Most French can speak English, but they'd rather swallow their tongue than admit it.
2nd choice: Rome. The city literally breathes history.Hectic but charming. Pay attention when you cross the road!
Only problem: Most Italians can speak English, but they sound like Mario and you won't understand them.
3rd choice: Berlin. It's "in" at the moment for some reason (or is it already "out" again? Don't know)
Only problem: Most Germans can speak English, but our English sounds as if we were marching. Kills all the romance. 
I would not recommend Venice. It's an obvious choice and probably one of the most beautiful, romantic and unique cities in the world, but it's overcrowded by tourists the whole year (keep in mind: Venice is waaay smaller than Paris or Rome and you'd stumble across other tourists every minute).
Definite first choice for me: Paris.
