I have always felt Charmander is the hardest to start and beat the game with. Brock usually is not the one that is the problem for me, though, because a Charmeleon with Ember, even though it is NVE, still puts Charmeleon's Special against theirs, and, depending on what moves they use, you can win a race against them (even moreso if you score a burn; I used to get Onix a lot). Misty was always the issue, as her Starmie is capable of blasting Charmeleon, Pikachu and Butterfree (often comprising of 3/4 or 3/5 of my team at that point when I played Red or FireRed) into next week with Bubblebeam (and later Water Pulse).
Even in the FRLG remakes, I would still say Charmander is the hardest. It gets Metal Claw to get past Brock, but Misty still puts it a very tight bind.
Squirtle has always been the easiest for me. The type coverage it can get on its attacks helps it get around types that should be strong against it. It could also use Dig for Surge and Ice Beam for Erika without going out of your way to overtrain or get the appropriate TMs. Someone earlier pretty much stuck their tongue out at type coverage, but that is a pretty ridiculous thing to do or to discount. Surf, Ice Beam, Earthquake and your favorite fourth move can almost trivialize the endgame (I liked Body Slam or Bite), let alone what it get through before that. There is a reason Squirtle is the starter chosen for a lot of the speed runs you see.
Outside of Charmander, Pikachu might be the hardest to play through the game with, because it has even less defense than Charmander, cannot evolve before Brock and has its best move at the time completely neutralized by Brock's Pokemon (meaning it has to use NVE Normal type attacks against the defense of Rock types). Pikachu is then neutralized again by Giovanni, even though it could have better moves by then.
Bulbasaur I have less experience with than the others, but being able to get by the first two gym leaders purely on its type and STAB, then being resistant against the next two usually gave me more than enough time to get it prepped for the endgame. He can have trouble with Blaine, Koga and Sabrina due to his types overleveled, but Venusaur can use Earthquake and Body Slam to great effect.
I can see and understand the arguments for any starter but Squirtle.