There was a study done that actually proved Squirtle is the best starter. Choose Squirtle for science!
I'm not so sure about Squirtle. Not because Squirtle is necessarily the worst, but simply that Bulbasaur or even Charmander are better choices, simply because you could get Poliwrath or Garydos. I will admit that Blastoise makes a good HM slave.
Rarity doesnt make a difference. Squirtle has the best defense overall against the gym leaders and elite 4 and the most offensive advantages.
Charmander may be cool but he is the weakest, he has the worst defense and his attacks faces the most offensive disadvantages. Also Squirtle is more versatile than the other two in terms of learning moves. He's the tankiest and best on offense even if hes not unique.
Bulbasaur is best against early gyms, Squirtle is okay against them but he is better later on and overall.
Fanath read the link I posted. Charmander and his evolutions are the weakest against the greatest number of gyms and elite 4 members. Squirtle is the strongest against the most. If the objective of the game is to beat the elite 4 (with a secondary objective being catching all of the pokemon) Squirtle is objectively and statistically the best choice.
Charizard is the strongest starter stat wise.  In regards to the gym leaders, char,Andre still wins. His high attack makes it easy to catch pokemon earlier on in the game, especially bug pokemon, which you can use later. We should remember that it isn't as if your starter is the only pokemon you'll use at the gym. Charmander can also easily gain exp by mowing down his opponents with his high attack stat. He becomes strong fast, and captures pokemon (That you'll use in the gyms) fast.
"Rarity doesnt make a difference. "
I'm sorry, but rarity DOES make a difference. You think most people are willing to spend a long time to find a growlithe, if one is even available in the game? Not to mention they have to have a fire stone. And the commonness of Poliwag and Magikarp simply makes Squirtle replaceable.
Squirtle may be replaceable but Squirtle is still better than Charmander. The fact that Fire types are rare doesnt mean that Charmander is any stronger vs gym leaders, he still has some major weaknesses. And Fantah your point proves mine, you wont use your starter exclusively, but the point is that you could with Squirtle pretty easily and you couldnt with Charmander without a big struggle. Also, Squirtle and Bulbasaur can capture pokemon as easily as Charmander (Bulbasaur perhaps the easiest with sleep powder) and their tankiness makes up for their slightly lower attack. Any pokemon can grind exp as quickly as any other, and killing wilds in one hit vs two hits doesnt make Charmander much better than Bulbasaur or Squirtle or any better at all since they can still grind just as easily.
No they can't. It's literally a fact that Charizard and his previous evolutions have the highest attack stat and the highest speed of the First gen starters, meaning he can hit harder and hit first. Taking in less damage doesn't matter if a Pokemon just knocks out the other pokemon before they can move. (Literally) My point in no way proves yours- Capturing pokemon is the goal. Not just using the freebie...
That really shouldn't happen. If you develop your team of 6 well, you should always be switching them out to give yourself the greatest type and stat advantages over your opponent. Remember, one strong pokemon does not a well-balanced team make. It is important to balance element types and battle types. (sweeper, fast striker, tank, status/healer, power, HM slave, etc.) No matter how strong Charizard is, a well balanced team can take it down. And besides, Bulbasaur might not be incredibly fast, but using sleep powder, leech seed and poisonpowder can allow it to take down a pokemon much stronger then itself. Equiped with an item that prevents a 1 turn kill, it is a valuable asset on any team. I will admit that Charmander would be my second choice, as Squirtle is replaceable by Garydos or Poliwrath.
I will admit that all 3 are good choices, but in general, Bulbasaur makes a better addition to the team.
Lordgrae's pokemon dream team:
Gardevoir (healer + sp. Attacker with decent health and def. Stats. Can learn electric type moves as well)
Hydreigon (fast striker, good type combo. Completely makes Charizard unnecessary)
Krookodile (amazing sweeper. Almost as good as Swampert. Is amazing with Dig and Earthquake)
Venosaur (status afflicter + tank)
Chancey, or whatever its new evolution is (tank. Heals self. Amazing def. Stats)\
Poliwrath (power pokemon. Wish it had higher def and health stats. Otherwise, obligatory water pokemon)
If allowed a second starter, replace last pokemon with Swampert
Extra pokemon to rotate through...
Garydos, Blaziken, Metagross.
Any other good backups to have (in case I know what my opponants pokemon are and I want to make a team more specified and less general)