Best Starter Line 1

I'm basing my vote purely on a Generation one lense. Squirtle has heavy water type competition to the point that it's outclassed by Gyarados(which you can get as early as route 3 with persistent grinding), Vaporeon(110 special with that bulk… granted after Rock Tunnel where it'd thrive but still has use after that), Tentacruel(120 special with the speed too!? Shame its weak to Psychic), Starmie(see Psychic dominance) and Lapras(is gifted late but if you know Gen 1 like I do, you can obtain it earlier 😇). Cloyster and Slowbro are also pretty good options.

Venusaur's saving grace is the Toxic + Leech Seed combo. Only one other Pokemon can even utilize it and it can't hit with Toxic as effectively since it's not a Poison type. Yet it's that typing that does hurt Venusaur(Psychic dominance) along with the fact that Electric does just as well against Water types.

The reason I picked Charizard or Charmander, albeit reluctantly, is because Fire types are really bad in Gen 1, and Charmander's movepool isn't that bad. It learns Fly(Yellow only) and learns Flamethrower reasonably early(level 38 as a Charmander...). My favorite or Vulpix is still the Queen at level 35 though with a better special as Ninetales but has a terrible coverage movepool(basically is Fire type or bust).

Post Gen 1, it's difficult to say but I'd likely lean slightly towards Venusaur due to the fact it counters Fairy types.

i would like to add that in gen 1, Charizard can also learn earthquake and dig to deal with giovanni’s partially poison/rock pokemon. the only one of his pokemon that charizard can’t land a super effective move on is dugtrio.

for some reason, giovanni’s only two ground moves are dig and fissure, which isn’t too surprising for gen 1. giovanni hates earthquake.

My vote is based on Squirtle being a turtle.