My favourites kinda lie in the starters of the generation. They're some of the ones I feel like have had a lot of thought put into them, but design wise and for the way they function in battle. Their designs may still be a little plain, although compared to some of the generation's other Pokemon, I feel like they're pretty good, and they're pretty much some of the strongest non-(pseudo-)legendary Pokemon in the games. This is the only region where I wouldn't ever consider getting rid of my starter for another Pokemon, just because I like them so much. It's not anything I'd be likely to do in any other generation, but the thought has crossed my mind in several others before. Treecko is probably my favourite out of the three, just because it's... so awesome. I can't really describe why, as honestly, I feel like it's gotten the most plain design out of the three, however I've always really liked that Pokemon.
As for my least favourite, I can't really say I have any. As I said, I feel like a lot of the Pokemon feel a bit uncreative and not really Pokemon-like, but I don't really dislike any of them to say, as none of them really are truly bad. I have no relation to any of them that'd make me dislike them, so none really.