In Gen1 itself, Charmander ('cause I always pick the fire starter). 6+, though, I love Mega-Venusaur.
Squirtle for its lowest form and maybe even second form, but I'm not a big fan of Blastoise. Charizard is awesome though, especially with both mega evolutions.
Bulbasaur is best base form, middle forms are about the same as each other, and final form Charizard slightly beats Blastoise with Venusaur in dead last. This is all in terms of design in my opinion. However, with Megas, both of Charizards beat Venusaur, who in turn beats Blastoise. So if this was starter evolutionary lines I'd choose Charmander, but since it's just base form, Bulbasaur.
Squirtle and its evolutions have always been my favourite. Not only is it adorable, but it also has the least weaknesses out of all the Kanto starters.
Gotta have to say Charmander, it was the first starter I ever picked after all so it'll always be the best to me.
I love Bulbasaur and it's evolutions, especially Venusaur. Charizard is pretty neat as well, but the amount of attention it gets is pretty ridiculous at times.
Because irony :wooper: I do like Bulbasaur and Squirtle, but I gotta give it to Charmander... It's hard to beat that nostalgic dragon xD
I can't speak for everyone else, but I chose Bulbasaur because of the type advantages it has over a lot of the gyms. Charmander just doesn't do it for me in Gen 1.
Bulbasaur. It was my first fully evolved Pokémon on LV, so that probably would tell you on its own. I always loved Grass types. And honestly, I think Bulbasaur is best in all forms, even mega. Then Mega Blastoise, then Mega Charizard X, then Mega Charizard Y.