Kanto: It's a toss up between Celadon and Saffron City. Celadon has the large department store and a game corner, plus it does seem overall more cheerful, but Saffron seems more realistic, with it's city of company buildings, apartments and houses. I'm leaning more towards Celadon though.
Johto: Goldenrod definitely. Home to Whitney and the first Normal-type Pokemon Gym (her Miltank was so bloody annoying). Plus, it's got a radio station and is the most city-like out of the region. I like cities.
Hoenn: My favourite region, and home to four of my most favourite-ist cities/towns/islands. Lavaridge for having the hottest Gym Leader, Sootopolis for just being unique, Lilycove for being a real city by the sea (can't beat that), and of course, Mossdeep for having the first double battle Gym and for just being an insland town with a giant space station.
Sinnoh: Jubilife for it being an actual city, Eterna for being more country-style living (plus Gardenia rocks), and Snowpoint for being a snow city.