Paris
Paris is one of the world's top cities by quite a few measures, and for good reason. There are amazing restaurants, tons of fantastic museums and galleries, lots of history, beautiful architecture, and great street food.
I've been there twice now, once while I was living in London, and then 4 years later when I was visiting my brother with my friend Andrew. I think I've seen most of the "must see" sites now, including the Eiffel Tower, Notre Dame de Paris, The Lourve, Basilique du Sacré-Cœur, and the Arc de Triomphe.
Beyond the sites, however, Paris is definately a cool place to spend an extended period of time. The general quality of life in the city seems quite high, and just being there is quite nice. As such, I wouldn't mind living there for a bit if the opportunity arose.
