Charmander... Charizard can learn EQ and RS, the 2 strongest physical attacks, and he almost always gets the first turn because of his speed. Immunity to earth is nice, weakness to ice/thunder/rock isn't... but thunder and rock pokémon are finished off before they get a chance to attack you thanks to EQ/RS. Having FT/RS/EQ gives you an advantage against all pokemon except psychic, water and fighting (although you don't need to worry about fighting since their attacks aren't very effective against charizard, and anyway they can be dealt with with fly, but I'm not much of a fan of HMs)
Bulbasaur as number 2 option; sleep powder + razor leaf is nice in pvm. Weakness to ice, psychic and earth isn't nice though. Exeggutor>Bulba. But Bulba is still great.
Squirtle isn't a very good choice, because like all water-only pokemon they are weak to thunder and leaf attacks, which are pretty common, and you only have an advantage against fire/rock (and leaf/flying if you leanr IB). The best water pokémon are the water/psychic ones.
But you can pretty much pick whatever you want if you're just going to pvm, the game is very easy.