This is quite different depending on whether you count my choice at the time the games were new or what my current choice would be. And there is the issue of me liking almost every Pokémon for one reason or another.
Gen I:
Back then, Squirtle no contest. One of my faves when I was a kid. I did like all 3 starters of course, but the choice was clear.
We've seen a lot more starters since then, and I realized after a few that GenI aren't THAT big of a dilemma to choose between, because they aren't as great as a group as most of the others. But I like them all, so by FRLG times my choices shifted to Charizard/Venusaur to balance out my Blastoise obsession (more Venusaur though, because Charizard has enough fans... a Pokémon being too popular seems to make me unfairly like it less xD)
So... if I started a Kanto-game now and had to choose... I guess I'd go for Squirtle.
Gen II:
Now this one is the first set that was extremely difficult from the beginning. I love them all so much, I think Johto-games are the ones where I restarted&traded-over all 3 starters to one game before starting the adventure the most times (although by HGSS times I would never have them all with me at the same time, but switch them out constantly, because there is too many other interesting Pokemon for having such an unimaginative ingame team).
I think Typhlosion was my favourite in GS-times just by a bit, very closely followed by Meganium (I loved the shade of green on the GB) and Feraligatr as well, as a worthy successor to Blastoise in my eyes.
What would I choose if you forced me to choose right now?... gah, I guess Cyndaquil.
Gen III:
I remember buying the issue of a magazine with Pokémon news in Austria, back when gen3 was starting to to come out in Japan.. and it had a whole batch of new Pokemon including the starters of course, but with their middle stages by that time as well. Holy cow. The perfect balance of simplicity of designs and the nice variation and diversity in personalities as well as the middle stages being spot-on, the perfect colour-choices that we haven't seen before (I mean Bulbasaur was turquoise, which really throws GenI starters as a set off balance) etc etc... but I was freaking blown away by Mudkip in particular. As the gen before, I instantly liked all three with all their stages, but Mudkip had priority, and Blaziken being least chosen, mostly due to popularity as with Charizard.
Weird, I don't even remember what I picked in my OR playthrough. Could have been either of them XD
Now I'd choose... geez, I have trouble deciding between Sceptile and Swampert actually.. but let's say Mudkip (even though its Mega is my least favourite lol)
Gen IV:
Oh gawd... similar to gen3, Infernape takes the backseat, with the tough choice falling between a flawless Grass/Ground tortoise and a majestic Water/Steel penguin x.x
In the games I'll have picked Torterra most of the time, due to not wanting to turn Empoleon into an HM-slave. Seriously, Gen4 was an amazing set, and felt like the complete opposite of gen2, what with each starter ending up with a fancy secondary type.
Torterra (sorry Empoleon and Infernape)
Gen V
Ah, the gen that was the closest to the original first Pokemon game experience so far (and well... the only one to even attempt that lol). Besides Emboar (people hated it for the repetitive combo by then, but I loved where they were taking that theme, and hey, no one complained about Samurott being the third pure Water starter, so technically this shouldn't be an issue ;D) the designs went more towards simplicity. The whole cultural theme was a nice touch.
If I had to choose... I'd rather not xD
But ok... Snivy
Gen VI
Ohh, the RPG-class starters
A lot of cool aspects about these three that make them noteworthy. I started with Chesnaught as I didn't know which of the other two would be more popular LOL. Feel sorry for Delphox now, when Greninja is obviously the "Charizard" in this case.
I was planning to choose Fennekin if there were XY-sequels coming out, so that's what I'm picking now.
Gen VII
Yeah, I'll pass until I have all the information about them. They're all interesting so far as expected, they each bring colours to their starter-type that I've been waiting for for a long time (Grass-not being flat out covered in green, Fire-using red rather than orange, but also employing black as its main shade, Water-a proper darker shade of blue instead of the usual lighter one).
In terms of types, I like that we finally have another starter that starts out with a (non-exotic) dual type in Rowlet. I kinda hope Litten does get Poison (seriously, how do we not have that combo in general yet??), but I also think it's way overdue for a Fire starter to be pure Fire again.
So...what is that together?
Water,Fire,Water,Grass,Grass,Fire.
Yeah.
As you can see, I don't "side" with any of the starter types without actually looking at the designs and everything. Neither of them are my favourite type thematically (which would be Ice, Poison or Electric...or Psy again now that its space-association is more clear), but even if one was, I'd try not to let that cloud my choices in a starter-set.
