The original three are still the best, especially Bulbasaur.
Nothing beats Bulbasaur!
Technically, I think Charmander is designed to beat Bulbasaur
According to Death Battle, Squirtle wins 48% of the time, more than the other two. Yeah defense stats!
I have to say, that after passing out with the other two, Bulbasaur’s poison immunity is invaluable.
Leech Seed also helped me a lot when my pokemons were so strong that any hit would kill an enemy that still had too much HP to get caught by my pokeballs. (I used it on other occasions too.)
Heck, I used that bitsize damage to wear out MewTwo, Articuno, Zapdos… Got every one in the first throw.