Goldenrod City, just because they're so much to do. Shopping, underground, training, dropping by the radio station (for contests), sight-seeing, getting my Pokemon haircuts, hitting the beach along the edge of the city, puzzles, games... Not to mention it's supposed to be the most festive city in Johto. Imagine what it might be like during Christmas, with all the streets lit up, snow falling softly, people bustling around, Christmas sales...
Second city I'd like to visit would be Cerulean, mostly for the water Gym, partly for the training in the area, and partly for fangirl reasons. (Favorite Gym Leader and favorite character in the same city -- two in one visit! You can't get any better than that!)
I can't think of any particular city I'd like to visit in Hoenn or Sinnoh. Maybe Verdanturf for the peace and Hearthome for the poffin-making house. No doubt I'll end up setting the latter on fire, though, so that might not be a good idea...