Lilycove - Awesome place. It's the home of the latest-appearing Department Store in the series(GSC's Celadon store doesn't really count since you already saw one in Goldenrod), and it might just be me, but there are a few better things than usual in Lilycove's Department because of it. The Contest Hall owns(even more so in Emerald now that ALL the contests take place there), trying to get all the painting for the museum is fun, and I really like the feel of the place.
Slateport - Great scenery, but aside from the Market and Battle Tent(which is fairly pointless after being able to go to the BF)/Contest House, there's not much to make me come back.
Olivine City - Slightly better than Vermillion, but still not very memorable. At least it has the Lighthouse and nearby Battle Tower(in Crystal, anyway)...come to think of it, I don't remember there being any sort of boat here that you can go on. Lilycove/Slateport have that boat taking you to and from the other, along with the BF in Emerald, and Vermillion had the St. Anne to g...oh wait, Olivine has a boat between it and Vermillion. I guess I forgot about that because the boat is fairly pointless once the Saffron/Goldenrod train is open to you.
Vermilion City - "It's bland, cramped, listless, and just boring overall. What more could you [not] want?" CB described it perfectly. :< The most interesting part of the city was the St. Anne, and that's only open to you for one event.
Gateon Port - ...this is in XD, right? As such, I have yet to see it. D: