My most favorite would probably be a tie between Kalos and Unova. Mainly because I'm a massive fan of what happened to the TCG during those eras, so that makes me biased. It's really difficult to pick any region, because they all have good memories with me. Reading the manga was awesome for me, and I particularly loved the mange for Ruby and Sapphire along with Diamond and Pearl. Overall, I'd say that Unova and Kalos just slightly get the edge, though. Of course, it's a very small difference, because I like all fo them.
My least favorite would probably be Alola. I don't know why, I just kinda don't like it. I know the anime had already gone down hill, but the art style switch-up definitely could have been better executed. The Pokemon weren't that original, which is to be somewhat expected as more ideas are used. In the TCG, it started the great era of re-using ideas. I realize that ideas have been re-used in the past. It's just that this feels like they're copying old ideas, rather than coming up with new ones or improving old ones. If I hear one more line about the uniqueness of Alolan Pokemon, I'm going to have to walk around with a delta species Pokemon constantly in my pocket. It's probably mostly just me finally seeing stuff being repeated from older stuff since I started playing. I understand that Pokemon is aimed towards bringing in new players with cool sounding stuff, but it gets annoying when they literally do stuff like bring back Lvl Xs and exs (lowercase intentionally).