I've been playing Pokémon Pokopia, and so I've been thinking about what regions in Pokémon are my favourite! My favourite city has always been Cerulean City. I usually pick grass starter Pokémon, but outside of starters, water Pokémon are some of my favourite. I especially love a lot of the original water Pokémon from the original 151, so I can't help but love the original water gym city too.
I feel like Vermillion City would have made a nice water Pokémon city too, given it's a port, so I also like Vermillion.