Kanto, for a really lame and predictable reason. It's not that I have some crazy preference to it because it's the first, I just know exactly where I can get some Nidoran. And so close to home, too.
The others make you work for it or they don't even exist. If I lived in those other areas iRL at my age, I wouldn't even leave the house. Maybe if I was 10-15 or something, cause I gotta be the best or whatever. But now? Poison bunny or bust.