Hmm, honestly I'm not too sure.

Barcelona is a great city, so's Madrid. But then London is crazy fun, and Berlin is amazing place at night. Paris is beautiful and calming. Dunno man, I love the place I live (copenhagen) but I reckon either London, Paris or Berlin would go for my favourite city in the world.

Although, if you're into that kind of thing, Karachi, Pakistan is pretty dope.