Hmm, for Kanto, I think I'd like Cerulean. It seems quiet, plus I love swimming, so a town with a gym that has a large pool would be great!
Johto would have to be Blackthorn City. (I think that's the one with Claire?) because of the cove in the back. Not many people are there, so it's be peaceful to just kick back and watch the Horsea.
For Hoenn I'm going to have to go with Pacifidlog. It's very out of the way, and it's a small team. Again, around water so you could just relax and watch all the water Pokemon.
I haven't played D/P so I can't comment on the cities there... I'm obviously a loner that loves the water ;)