Hoenn for 3 reasons.
1. Kanto is almost every generation.
2. Hoenn is left alone so it doesn't make the games suck because of the nerfs. Don't get me wrong, I don't hate Hoenn. I love Hoenn, but that's actually a good thing Game Freak didn't use it outside of R/S/E because Kanto as a second region made G/S/C my (second to if it has to be the version of the region I live in) least favorite. Sure it got fixed in the remakes such as HG and SS, but I digress. If you're gonna have a second region, then make it good. Otherwise, just stick with only one region.
3. R/S/E (Yes I'm always gushing about it) is my absolute favorite of the series.