Bulbasaur is strong against all of Brock's and Misty's pokemon, and resistant to Surge's and Erika's, making it a very safe strategic choice against the first four gyms. By the time you reach any of the gyms that it will have a problem soloing, you will have had plenty of time to find different types of pokemon to cover its weak points.

But the correct answer is still Charmander, despite being at a terrible disadvantage against both of the first two gyms. Those two gyms cover all of Charmander's weaknesses. Once you've managed to defeat them, it's smooth sailing from then on.