The lights are out, you're sitting close together, the movie is the right mix of emotional and uplifting. I'd have to say that sticking with the tried, tested and true: the romantic comedy is the best way to spend the evening with your loved one. Maybe it's cliche, but why fix what ain't broke?
I'm not too big of a fan of romantic comedies however, so the best are the ones that appeal to both genders, instead of making one of the two want to stab their eyes out.
I don't know if the first two movies have been released on DVD yet, but Playing By Heart has been available for almost a decade now, and it's really good. It's also pretty unknown so it'll make a nice surprise for your girl.