Kanto - the base forms are too similar. They all share the same face, so much as you could colour charmanders head in blue and it's basically be squirtle. Doesn't help bulbasaur is blue like squirtle too, lacks colour diversity, all based on reptiles.
Johto - They're all cute, though I wish Cyndaquil stuck to more fire based colour palette. The final evolutions are a bit generic.
Hoenn - Great, diverse species choices. Represent their elements well. Evolutions are creative and stick to the original concepts. takes 2nd place.
Sinnoh - can't be beat. Diverse species choices with all evolutions looking badass. Colour compliment each other well and fit in with their region. Diverse body shapes and features, based on legends. Takes first place.
Unova - Pig ruins it by beung uncreative and another fire/fighting. No idea why Samurott is not standing up, and Serperior is a bit generic.
Kalos - start of the slim/macho/feminine approach to the starters. I guess the idea is that there will be one for everyone, and while the first forms look great, delphox and chesnaught look like people in suits due to their humanoid appearance.
Alola - same problem as Kalos except way worse as they took the slim/macho/feminine concept and went the whole ten yards with it, delivering the most furbait starters to date. Shame, as the first forms are the best.