Venusaur is hands down the best. My personal favorite is Charizard for his majestic design and the buffs he receives in later gens, but that is not what the question is about. Its who has the most advantages overall in this generation and objectively that is Venusaur.
Because of the gen 1 mechanics treating special attack and sp defense as one stat Venusaur is better offensively and defensively than Charizard or Blastoise. Venusaur is essentially battling with 100 in both stats. This mechanic worked to the disadvantage of the other two, leaving Charizard and Blastoise stuck at the average 85. Venusaur could put out a lot more damage than either one of them, and continue to increase that damage output while taking less damage in return through the move Growth, the Calm Mind of the day.
If you weren't running set up you also had the alternative of Razor Leaf, which is always a crit hit when it lands because of the different formula based on speed that was used to determine critical hit probability. Since the damage was nearly doubled this effectively gave Venasaur a stab attacking move to abuse that rivaled hydro pump and fire blast in damage but with higher accuracy of 95. Venusaur has also lots of utility options from recovery moves to Swords Dance to almost every kind of status.
In addition Bulbasaur evolves at lower levels than the others, which gives the player acess to his 2nd and 3rd stage evolutions first, making life a little easier.
He even has the best type matchup for the structure of the Kanto region. It might seem like its Squirtle because the pure water typing is generally more resistent being weak to only electric and grass, those two weaknesses do happen to have gyms relatively early on though. While Bulbasaur is weak to the same number of gyms two, he has more direct advantages, resistances and immunities. Squirtle's line is super-effective against Brock, Blaine and Giovanni while resisting Lorelei and Misty. The Bulbasaur line is super-effective against Brock, Misty, Giovanni and to some extent Lorelei, he resists Erika, Bruno, Lt Surge and the fighting Dojo and is immune to being poisoned by Koga and Team Rocket.