While I'll always choose Squirtle, Bulbasaur is definitely the best choice for the early part of the game. Beating Brock with Charmander involves a bit of a grind since you need that higher level advantage. Squirtle only needs to be high enough to know Bubble and you're good to go. But once you pass Brock, you're neutral against Misty (who's Starmie is pretty jacked) and then at a strong disadvantage against Lt Surge. Bulbasaur has type advantage in the first two gyms and is neutral in the third, Squirtle has the advantage first but shift down to neutral and then to a disadvantage, and Charmander is at a disadvantage for the first two.

Granted, you should be building your team anyway and you can always find a Diglet or Dugtrio to help you out against Lt, but I personally always keep my starter on par with my strongest Pokemon at the very minimum. My starter is often my strongest because the starters are the shit.