I think it could be either Mauville or Lilycove. Mauville is at the center, which is where a lot of capitals are located in, but it doesn't really have much outside of a game corner and a gym. Lilycove makes more sense because of it's size and because it's got the Dept. store and the pokemon contest.
Rustboro and Slateport also make sense to me just because of their size.