You can choose any pokemon which you love, but from my point, Charmander is considered the best. If you choose Bulbasaur, then your little dinosaur will face a FLYING and FIRE lizard in the end, which is quadruple the power to a tiny GRASS pokemon. If you choose Squirtle, you can destroy Venusaur later with your Ice Beam, but later in the game you will find more powerful Water pokemon. Yes, more powerful. For example, exchange with your friend and have a Polywrath - Fighter pokemon, or Gyarados or even Lapras. As you can see, lots of Water pokemon in the First Generation to choose. However Charizard is the only Flying combined with Fire, with great Special attack and dmg.