I feel like I see a fair amount of route threads but never a thread about people's favorite town/city in a Pokémon game, so I'm gonna change that.

For me, its Sootopolis.
I remember exploring the whole sea before getting Dive, and being really confused about how to get to Sootopolis. So later on, when I was combing the ocean floor with dive, I was floored when I found a cave entrance that took me to the inside of this crater-dwelling City. In general, Hoenn does a really good job at making a lot of its settlements feel very remote, like they're hidden away from the rest of the world, and Sootpolis is probably my favorite example of that. Discovering Sootopolis felt so amazing because before this point, exploring the ocean floor wasn't something you could do in pokemon games, so its not the sort of thing you'd see coming, and as such its discovery feels so much more impactful compared to, say, if it were just an island city like Mossdeep. And the entire essence of the city, from its aesthetic to its music, feels suitably distinct from any other place to match; https://www.youtube.com/watch?v=rFX8tILtNE4