Oooh, this is tough one. Well, no, not so tough, so long as I don't have to pick only one.
I'm gonna say Japan has to be up there on the list. If you want a specific subculture, then the overlap of expat culture with Japanese culture. Obviously this is something I know pretty well, and I think it's full of funny or poignant stuff that the general public seems to find interesting, so I'm excited to share it with people.
But I also like making up cultures. This goes along pretty well with the worldbuilding question from the other day. I'd say that to the extent I do worldbuilding, it's more about sharing a culture with someone, with a few extra details that I have to throw in because they don't know that culture already from their own experience.
I really, really enjoy working in the Old Somneran culture, in Altitude, and how that evolved into New Somnera / Novera. That's all about a culture clash, so of course writing the culture is fun. I have a lot of background on the Somneran culture -- and the underground-dwellers, Those Who Live in Holes in the Ground, as a not-nice way of putting them. Cultures in general are fun to write about; it's amazing how much they're like characters themselves in a way.
Then there's another variety of culture -- say, high school culture and all its subcultures and cliques. That's something that tends to get into my writing a lot. I've realized lately just how much of our personalities seem to form in high school, and even when we're long out of it and no longer in the roles we got fit into back then, we still think of ourselves as members of that culture, even if other people don't see us that way. So the idea of what you are vs. how others see you is a fascinating one -- and that can go between national cultures too, for sure.
Got a story on this subject open in another window. Gonna go work on that now. :)