I think US major cities still have very active players and groups...
As far as "funnest" cities to play in, I love playing in Hong Kong and Singapore (I travel between these two countries/cities very often). These 2 places have very diverse biomes depending on where you go in them so you can catch Pokémon of every type! Lots of parks with nests!
Gym-wise though, super competitive. I was at a gym where the minimum CP was 35xx! Of course full of D9 and Rex only so easy to take down with Lapras and Machamps.