For romance, I'd choose...
Heidelberg, with its gothic ruins of a castle overlooking the town and the river that runs gently through it
Venice, with its countless nooks and niches, surprising nearly secret squares, bridges, crumbling edifices and gelato vendors
Kyoto, and all those spectacular gardens
Paris, at dusk there are few cities that can match its sparkle, charm and beauty
The 5th is a tossup between many wonderful cities where I've experienced romance. For variety, I'll pick Lucerne. It's photo-worthy in any season, but unlike many cities, is also beautiful in the wintertime.