Who is the best starter Pokémon in the origional games?

they are far too over popular

squirtle 6

Charmander 4

Bulbasaur 3

I voted Charmander but I only voted for him because he has objectively the fewest weaknesses compared to the other two but the answer is none of the above and I'll explain why. After having played Pokémon Yellow for ~25 years, I've concluded that when it comes to Generation 1, none of the starters are even necessary despite the fact that in Yellow version, you can obtain all three of them as gifts. To defeat Generation 1 with relative ease, you objectively need to have a solid Electric type, a Ground Type and a Water Type. Rest of the team can be w/e you want or need to get through the game.

Case against Squirtle: Now you may be pointing out that by including a Water Type in my mandatory Type list that it would make him the best Starter. While he is indeed strong to start off, he's massively hurt by two things. Firstly, the fact that his Physical movepool is hurt by his poor speed. He doesn't learn Bite until Level 22(Squirtle) or 24(Wartortle) and the only other Physical moves of note he can learn are Dig, Body Slam or Mega Punch. Dig would be solid coverage against Electric types but given his poor speed, you're going to get zapped and KO'd before you're able to use it(plus its best on a Ground type like Sandshrew anyway). Body Slam would work but it doesn't come until S.S. Anne and even then, I'd much rather have it on Tauros or a Normal Type for STAB. You do get Mega Punch relatively early but its poor accuracy makes it a bad move in my book. There's also Strength but it's an HM and should only be taught if you got no one else that can learn it.

The other major thing hurting Squirtle is the plethora of Water Type options in Generation 1. The first one that stares you in the face is Gyarados. You can obtain Magikarp from the Salesman so early that if you dedicate yourself to grinding it to level 20 for Gyarados, you can have an end-game powerhouse with a 125 Base Attack Stat w/ Bite to steamroll Misty if needed. Gyarados is also faster than Blastoise and has a much stronger Special. Then there are other notable Water Types in Vaporeon(learns Aurora Beam in Yellow), Starmie(Water/Psychic anyone?), Tentacruel(best Speed + Special for a Water type bar none) and Lapras(gift Pokemon that requires substantial grinding but Water/Ice is pretty nice with Ice Beam at level 38). Even Slowbro is an option thanks to Surf + Psychic + Amnesia.

The lone thing Squirtle has is that he has a pretty thick Physical Defense but against opponents who'd normally deal Physical Damage, you'll generally outspeed them while dealing x2 or x4 Damage in return and if its Physical Defense you want, Cloyster has that in spades while having STAB on Ice Beam.

Case against Bulbasaur: Bulbasaur, as a Grass type, has a really nice start but the thing that plagues him is that w/e Grass is strong against, another Type does just as well against it. Grass has a x4 advantage against Rock/Ground, Water/Rock and Water/Ground. Against Rock/Ground, Water has the same advantage, hence why Water is among my mandatory Type picks. Rock/Water exists via the Fossils, but you never face them in-game, something I felt the creators missed with the Rival as I would have replaced Rhydon with one of the Fossils based on which one you pick at Mount Moon(making the choice consequential and boosting Grass viability for that reason alone). Finally, Water/Ground does not exist in Generation 1, hence an Electric Type can shock any Water Type foe without fear of having it neutralized by that Ground typing. As a result, an Electric type does the job of Bulbasaur at the same effectiveness while being able to zap Gyarados for x4 damage(which you do face in E4). Grass is also resisted by Poison Types, something that Water and Electric do not have so against Tentacool/Tentacruel and the Nido's, Grass is at a disadvantage. He's also not the only Grass Type that can learn Razor Leaf. Bellsprout(Blue/Yellow version) can learn the move, albeit later than Bulbasaur can but has a much earlier Sleep Powder. Sleep moves are quite important, particularly when it comes to catching Pokémon and if it's a Sleep move you want, Paras is the Grass Pokémon you want to use via its unique Spore Attack that it learns at level 28.

I must also point out that being part Poison can be a curse in Generation 1 due to Psychic dominance. With that in mind, you're better off using a Grass Type that doesn't have that vulnerability(Exeggutor or Parasect). Plus, Bulbasaur is only able to learn one HM in Cut, a notable downside. He is the only one that can reliably do Toxic + Leech Seed but outside of that, he doesn't really have anything going for him. Oh, and fighting your rival's Charmander early on can be a real pain without an early-game counter that the other two picks have.

Case against Charmander: I may have chosen Charmander, but I did so only by default because what plagues Bulbasaur, Charmander also falls into the same category. Namely w/e Fire is strong against, so too is another Pokémon Type. Fire shreds Bug, Grass and Ice. Bug is plentiful early but so too is Spearow, a Flying Type with Peck that can x2 them with ease. Charmander might burn Erika and the Grass Type Gym down, but Fearow and Dodrio can both utilize Drill Peck to pierce their Defense(typically the weakest stat of Grass Types w/ Tangela being the exception). Even Parasect, as a Bug Type, has x4 via Leech Life on Erika's Pokémon(w/ Spore to put them to sleep). As for Ice Types, 3 of the 5 you'll face also have Water Typing, making Fire moves neutral while inviting a Water Type retaliatory strike. Only against Jynx and Articuno will you deal x2 Damage and even then, you're rolling the dice because both happen to be very fast for Ice Types and deal x2 Damage in return since Fire doesn't resist Ice in Generation 1. Might be better off rolling with the Speed freak Aerodactyl and using Rock Slide to deal x2 on their Defense(x4 against Articuno).

Now you may be asking yourself, if other Pokémon can do the job of a Fire Type, why did I pick Charmander? I picked Charmander because when it comes to Fire Types, he's arguably the best one overall. Neither Arcanine or Ninetales can learn Strength despite being version exclusives(w/ Growlithe learning Flamethrower at level 50) and both have limited movepools. Magmar as a Blue exclusive can learn Strength but because he comes so late with a level 55 Flamethrower, he kind of sucks. Rapidash and Flareon need no introduction as to why they're bad and Moltres is a legendary on Vicotry Road without Flamethrower. Therefore, by default, Charmander is it despite the fact that Vulpix is my favorite Pokémon and Ninetales having the best Speed + Special of all non-legendary Fire Types with the earliest Flamethrower to boot(level 35 compared to Charmander's 38).

Charmander's physical movepool is similar to Squirtle, with Slash at level 30 and enough Speed to use Dig to cover against Electric and Rock Type Pokémon. He's even better in Yellow version because he's able to learn Fly as Charizard and take advantage of his Flying Typing while also having Strength and Cut as options. Three HM moves in Yellow makes him a solid HM slave if nothing else while being a capable counter to Lorelei's Jynx. Yet, as I've said above, he's not a must-have option. Only the last one left standing given the options available.