Paris, Paris, Paris, I can seriously never get sick of this city. Every building is beautiful, the metro is easy to navigate, there are crepes and croissants everywhere; it does not get much better than this. I suggest staying near Trocadero for more quaint or Latin Quarter for a younger crowd.
One of my absolute favorite places in the world is Sacré-Cœur. There are a whole lot of stairs (sure is a workout) but when you hear the harp playing, you look back and see all of Paris, and you take in the atmosphere, you'll most likely have this great feeling that I have every time I am here!