Favorite region? Sinnoh. Especially in Platinum, where your adventure starts once the snow melts and the Battle Zone is comfy as heck. The only problem in Sinnoh are bogs and defog being an HM. Unova is also nice with its comfy urban setting, but I'll end up liking it more than Sinnoh if it's more advanced and they did more with the Battle Subway (like actually moving to another city once you get into a station) and Mistralton City (I was wishing they'll make it a huge airport city with farms surrounding it). If Unova will have full and advanced roads and cities in the BW remakes, maybe I will like it better than Sinnoh. Also, Alola is growing on me so far. I'm excited for Sun and Moon!
My favorite game is Pokemon Platinum. It has a cozy region, amazing post-game, and just the right amount of challenge and story. White 2 could've been my favorite game but I gave up on it (didn't finished) because it's too slow and there's too much dialogue. Pokemon World Tournament could've been nice.