This was probably the hardest poll I've answered since joining these forums.
Pros: Nostalgia. All start out cute, yet all turn out to be badasses. Their evolutions are simplistic, and easy to follow.
Cons: They're all reptiles, more or less. They all shout the same exact personality, such that ultimately typing is the only reason to choose one over the other. Bulbasaur starting off as a grass/poison is a wrench to the pattern of all starters being monotyped.
Pros: They all have very distinct personalities: Chikorita being feminine, Cyndaquil being timid, and Totodile being aggressive. To this day, Meganium is the only fully evolve starter with a huge grin spread over its face.
Cons: Their evolutions all had boring types. When people are polled on which starter they'll choose, this trio has the biggest imbalance, with Chikorita usually only getting like 10%.
Pros: All evolve into badasses. First generation in which starters had (temporarily) signature moves. Very diverse in their battling tactics: Sp.Atker with amazing speed, Mixed Attacker with average speed, and Physical Tank.
Cons: Torchic's evolution was too drastic, al beit awesome in its own right. Lacked personality.
Pros: All very distinct species--a reptile, mammal, and bird. All had fairly unique secondary typings that weren't forced. Referenced mythology (the earth is a tortoise), literature (Journey to the West), and history (Napoleon). Made use of the phys/special split to be even more unique in battle strategies.
Cons: All very drastic evolutions. Didn't have much personality in any of their appearances.
Pros: The most distinct personalities. Snivy was immediately dubbed "smug"leaf by the fandom, Oshawott has a frowning face, and Tepig has a jolly face. Snivy becoming royalty perfectly fit its personality; Emboar continues referencing Journey to the West; Oshawott being a samurai-in-training is cool. These all gave factors to choose from other than just what type you like most.
Cons: Tepig and Oshawott's evolutions were unnecessarily drastic for starters.Water and grass had the least number of dual-typed starters compared to fire as of GenIV, yet somehow fire managed to be the only type which got a secondary typing. Speaking of the fire starter's secondary typing....
Pros: Very vibrant, and distinct species from one another (rodents, foxes, amphibians.) Chespin being mostly brown, and Fennekin being most yellow, is very refreshing when starters are almost always green/orange/blue.
Cons: We've already had 2 fire-type foxes: Ninetales and Flareon. We've already had 2 water-type frogs: Poliwag and Tympole. I would argue that these are the first starters with redundant species/type combinations.
Ultimately, I chose Kanto.