I'm not one to care about stats and type advantages and planning all the way through the game so I usually pick based on looks or if it's the underdog of the other starters from what I've observed.
Kanto: Squirtle, it was my favourite for a long time and I couldn't think of picking anything else, okay maybe Bulbasaur but I find it hard to freely pick Charmander.
Johto: Chikorita, absolutely adorable and Meganium was somehow the first Pokemon I knew about properly and was my favourite before Squirtle. Plus it is sort of the underdog for Johto.
Hoenn: I've never had an overbearing opinion on any of the Hoenn starters so I can't really say, they all look nice in all evolutions.
Sinnoh: For my favourite generation I couldn't pick a favourite starter, they're all great to me, I have all three generation four games and on each of them I've chosen a different starter to be able to play them all.
Unova: Just based on looks, Oshawott because it is rather cute in its own special way.
Kalos: Chespin, I love that little spiky ball-rodent thing. On my first play through I had no hesitation in choosing it, I'm not so keen on Froakie but its other two evolutions are good. Haven't yet played through with Fennekin and I probably won't until the third game comes out because I don't want to restart my game again (I have my Raichu to blame for this).
So yeah, starter choice is purely aesthetic and I will admit that the anime has played a part in some choices (Squirtle and Chikorita, you amazing little rascals).