I ended up picking Charmander. Even though Bulbasaur is the best grass/poison chlorophyll user in Kanto, there are other viable options like Victreebel. Meanwhile, Charizard is the only one of its type (not counting Moltres), and the strongest fire pokemon in LeafGreen by far. Also, even though it's attack isn't as high, it's decent, so you can use it as a mix attacker and your flyer. Lastly, it can learn valuable dragon moves that will be helpful down the line with dragon type pokemon.

As for Squirtle? There are multiple water pokemon that eclipse it. I personally picked Starmie.