Count up all the instances where each of the starting Pokémon have the upper hand (or at least an advantage) against the 12 gym leaders, and you find that while Bulbasaur might do great at the beginning of the game, it also will have the hardest time of all the Pokémon finishing off the rest of the leaders. Squirtle, on the other hand, has simultaneously the most advantages and the least amount of disadvantages against the leaders. Squirtle and its evolutions come out on top.
You shall choose Squirtle. This thoroughly researched article on Scientific American explores which is the best Pokémon to start with. But they got something wrong: fire, Charmander, simply is cooler.