Bulbasaur vs Charmander
I'm assuming this isn't for competitive and just gameplay-wise.
Bulbasaur
Pros:
-Beginning of the game with Bulbasaur is fairly quick and easy - Bulbasaur has the type advantage in the earlier gyms.
-Will gain nice HP and defence stats eventually; this coupled with moves such as leech seed and poison/sleep powder can make Venusaur an effective tank.
Cons:
-Towards the ending Venusaur might start to struggle. It doesn't have a brilliant type match-up against the E4.
-Tanking in-game could be tedious
-Quite a limited learnset (can be remedied with TMs but could be better)
Possible Replacements :
Vileplume : Same typing and similar play-stye
Exeggcutor : Obtainable quite far in the game. But has a nice typing and bulky enough to take hits
Charmander
Pros:
-Wider movepool than Bulbasaur
-Hits hard and fast thanks to it's great speed and attack( Charizard has the highest SPE and ATK stats out of all the other starters
-Will be fantastic against the E4
Cons:
-Starts with the lowest stat total and average
-At a disadvantage type-wise for earlier gyms
-Charizard is frail compared to other starter (glaring 4x rock weakness)
-Only learns two useful fire-type moves, ember and flamethrower. While ember does come at an early lv9, Charizard gets flamethrower at lv46, which is a bit cumbersome
Possible Replacements :
Arcanine: Solid and reliable. Easy to obtain in Red
Flareon: A bit slow but otherwise well rounded
Overall, there is no 'best starter,' as such, and it is really up to you and your play-style. If you like tanking and slowly whittling your opponents down, Bulbasaur will serve you well. If your prefer something more fast and harder-hitting, then Charmander's up for it.