Let's rank 'em!
#6 - Sinnoh. I like Turtwig's line, though it's not my favorite. It's pretty cool. Piplup is cute, but his evolutions are awful. Empoleon might be the most over-designed Pokemon there ever was, it's hideous to me. And I dislike monkeys so there wasn't really much appeal to begin with Chimchar's line for me.
#5 - Hoenn. Hoenn is my favorite region and there are no starters I dislike here, but if I have to rank them in order then this is where they have to be. I kind of like that Gamefreak challenged themselves. I mean, you have rams and tigers in the Zodiac but they chose a chicken? AND managed to make it look great? Kudos! For their semiaquatic animal they chose a mudfish? And, again, they made it look really cool! I'm not a big fan of Treecko's line, but they're not bad by any means.
#4 - Unova. Tepig sort of ruins this for me. I think it's a great design, actually, just not something that's appealing to me personally. It's pretty clever to merge pigs and some sort of sumo warrior thing xD Snivy and Servine are okay, but Serperior is awesome! I love snakes! Oshawott's line is fantastic all the way through, and Samurott is a beast! I'm wondering why it's not Water/Fighting, though...
#3 - Kalos. Nobody's seen the evolutions yet, of course, but I like these little guys already! I disliked Froakie at first, but it started growing on me. It's so fluffly and cute :3 However, I prefer Chespin and Fennekin. Both of them look both adorable and like they could be monsters in battle.
#2 - Kanto. The classics! I totally dig reptiles, so I'm fine with them all being based off of those animals. Charmander and Bulbasaur are some of the most awesome Pokemon there are. There's not that much to say, they're just great!
#1 - Johto! CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA HAM CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA CHIKORITA! Also Cyndaquil and Totodile are pretty cute :3