A lot of people would just pick Charmander because its the most popular, but lets not judge by popularity, lets judge by stats!
Charmander: Charmander's base stats are garbadge, but hey, what can you expect from a starter? When he evolves to Charizard, its no more playing around. Charizard's special attack is very good with a base 109. Its speed is also very good with a base 100, and the rest of its stats are pretty decent. His stats total is 534.
Bulbasaur: Bulbasaurs stats are trash, so enough of him. When it evolves to Venusaur it get much better. Venusaur's special attack stats is a nice ol' 100, so is its special defense stat, and the rest of its stats are all around great. His stats total is 525.
Squirtle: Squirtle is a starter, so his stats are not so great. When he evolves to Blastiose he gets much better. Blastiose is a bit more interesting from the last two because its attack and special attack are pretty equal, so you don't have to worry on what moves to give him. His defense is great at a base 100, and special defense at a base 105. His stat total is 530
Comparison: So all these three are pretty good Pokemon, but when you look at Charizard, he is the fastest and has a great special attack stat and has the highest stats total, so I would recommend Charizard to be on your team!
I hope this helped!