99% of us are going to say Hiun, aren't we? D:
Truth be told, I don't like most of the cities in Black and White. I love the music for most of them, but nearly all of the cities themselves are incredibly bland and boring.
The only two that stand out to me are Hiun and Raimon. Hiun isn't much more than a generic city, but it gets points for abusing the hell out of the engine. It does look amazing, and while it isn't quite as big as I hoped it'd be, it's still pretty huge.
My favorite city would have to be Raimon, though, without a doubt. Awesome music, tons of things to do even after post-game (which is one thing Hiun lacks), and while it's not huge like Hiun, it's still pretty big. And I just love the atmosphere of it, too. It's just a very fun city, really.