What? Your listing is more than a little odd. Charmander is no top-tier, if you have to overlevel him to beat Brock...
You don't have to overlevel Charmander. Use Ember. It does decent damage against a Geodude and Onix. Use Growl if they use Bide.
Easy match. Want proof?
Watch a level 11 Charmander beat Brock's lvl 12 Geodude and lvl 14 Onix.
And the Elite 4 member he should be good against, Lorlei, is actually a real threat to him due to most ice types being water as well.
Charizard beats Lorelei easily.
Watch Charizard steamroll Lorelei with Fissure.
I agree that Squirtle is quite good once it learns Ice Beam and Surf, but your point on Fissure is pointless. You could essentially say any pokemon who learns Fissure just needs an X-Accuracy and an X-Speed to kill everything, so what makes Squirtle so special in that regard?
Bulk, typing, availability. Feel free to
browse the list of Pokemon that learn Fissure. Find another pokemon that has Blastoise's availability, and the typing and stats to setup Fissure against the final bosses. I'll give you a hint: You can't.
If he needs items to set up, then it shouldn't be considered. Otherwise I could argue that I should give Rattata Hyper Beam and then jam X-Speeds and X-Attacks down its throat as it goes on to KO everything. Or that my level 1 Pidgey is great at beating the Elite 4 because I used 1000 revives on it.
You're comparing using 1000 revives to using 1-3 X Attacks and X Speeds?
Really? Be practical. Go back to
watching Charizard steamroll Lorelei. He used 1 item, and then 5 attacks to win. Charizard beat a boss in 6 turns. Underleveled.
Now imagine a pokemon that can do that to nearly every final boss, and at an even lower level. Oh wait. You don't have to. Just use Blastoise and Dewgong.
And who says "using items shouldn't be considered"? You? As long as items can be easily purchased and used within reason, they're fair game. Do you ever use a couple Super Potions?
Bulbasaur needs more credit than you give him. He gets Razor Leaf, which is essentially the best grass move in the game, and he gets it really early. ...
No. Ivysaur gets Razor Leaf at 30.
...and he's very easily the best out of the starters. He has access to Sleep/Poison/Stun powder on level up and he gets them early
No. Venusaur gets Sleep Powder at lvl 55. Meanwhile, Blastoise and Dewgong can solo the final battles at lvl 42. Venusaur never learns Stun Spore either.
Also Bulbasaur is the worst starter. Bulbasaur being good early game is a myth.
Bulbasaur is NOT good early game. He's super-effective against two Gym Leaders. All that means is he's great in a grand total of... two battles. By the time you've reached Misty, you've probably had over one hundred encounters. I don't care if a pokemon is great in 2% of his early-game battles.
What about that other 98% of battles? How does Bulbasaur do? He gets owned by Zubats in Mt. Moon hitting him with 4x effective Leech Life. And he slogs through multiple Bug Catchers with Tackle.
Bulbasaur is terrible attack-wise. Grass/Normal attack coverage is awful throughout the game. Especially with all the Flying, Poison, and Bug types you'll be running into through the life of the game. And don't even get me started on how useless Ivysaur is at Lavender...
If you really want a Grass type, just use Victreebel for the reasons I noted earlier. At least Victreebel can abuse Sleep Powder at lvl 18, and Wrap things safely.
Alakazam is good. Can't argue with that. But a low 40s Blastoise/Dewgong can solo final bosses in the low 40s in 6-7 turns with Fissure or Horn Drill. What about Alakazam?
Nope.
Top 4 In-game Pokemon in RBY are probably:
1) Blastoise
2-4) Dewgong
/Nidoqueen
/King
And Alakazam is still way behind stuff like Charizard and Sandslash.
I pretty much stand by
my original tierings except for maybe Clefable(who still trumps Venusaur). If you're still unsure about my top tiers: Blastoise, Dewgong, Nidos, Sandslash, Zapdos, and Charizard, then I recommend actually doing an RBY run through. All of the above pokemon are superior to your opponents from the moment you catch them til the end game.
All this theorymonning is useless compared to actual experience from a runthrough.
Edit: Also, to those mentioning Farfetch'd as overlooked, you are spot on. Farfetch'd, Mr. Mime, and Seel/Dewgong are all fantastic in-game trades. They're also always severely underrated. Seel/Dewgong is top 2. Farfetch'd does amazing damage with Slash. And Mr. Mime potentially trumps Alakazam as an in-game pokemon since Mr. Mime can learn Thunderbolt.