Paris-- culture, food, wine, people, ambiance. I absolutely love walking the streets, pretty much aimlessly, for hours.
Prague, London, and Barcelona are my and my wife's other favourites, but Paris holds a very powerful place in our hearts. And San Francisco, for exactly the same reasons as Paris, is high up there as well.
That said, we're looking at Antarctica at the end of this year. Gonna eat me some penguin!