Charizard Fire Blast vs. Venusaur: 236-278 (65 - 76.5%) -- guaranteed 2HKO
Venusaur Razor Leaf vs. Charizard on a critical hit: 31-37 (8.6 - 10.3%) -- possibly the worst move ever
Worth noting that toxic gets downgraded to regular poison after switching out, which does 1/16th of damage every turn, and blocks other status like paralysis or sleep.
Leech seed would probably not do much, and is lost upon switching out.
Growth is ok, but since venusaur is slower, you only reduce damage by 33% on your second hit, not great.
Sleep powder would give you a chance, if it hits (75%) and if zard doesn't crit. But it would also waste your team's sleep if you are playing with sleep clause.
Q.E.D Charizard>Venusaur
That said it's not a transitive superiority, as we know Blastoise> Charizard, and Venusaur>Blastoise.
That said Charizard beats venusaur harder than venusaur beats blastoise, as the turtle has access to ice beam and blizzard, so it can at least deal non-stab super effective damage back before getting wrecked by razor leaf. And as a bonus gets a chance to crit or freeze.
Blastoise Blizzard vs. Venusaur: 158-186 (43.5 - 51.2%) -- 5.4% chance to 2HKO
Venusaur Razor Leaf vs. Blastoise on a critical hit: 256-302 (70.9 - 83.6%) -- guaranteed 2HKO
Finally while Zard doesn't have SE coverage moves against Blastois it is faster, so it can get 2 moves in before dying, either 2 slashes or a swords dance and a hyper beam, which deal the same damage but would leave zard roided against the next mon. (Or a slash +beam if you roll for a crit.)
So while mon superiority by matchups are not transitive, that is, we cannot order starter pokemon from best to worst. We can definitely order their matchups from best to worst:
1) Zard vs Venusaur
2) Blastoise vs Zard
3) Venusaur vs Blastoise
With 2 and 3 being closer than 1 and 2.
The matchups themselves are transitive, and as such, in terms of starter matchups Charizard is the best, and Venusaur is the worst
> Worth noting that toxic gets downgraded to regular poison after switching out, which does 1/16th of damage every turn, and blocks other status like paralysis or sleep.
> Leech seed would probably not do much
No, that is incorrect and exactly my point of the argument (and therefore the reason why you would've failed my website's captcha check).
The first editions we're talking about had a bug which meant that if Toxin is used first, then Leech Seed does an additional damage per round. And it's ever increasing because the multiplier of Toxin was applied to Leech Seed, which no defensive attack can counteract except Rest (and a Relaxo that had enough HP to survive a second attempt of Toxin+Leech Seed stacking).
Note that leech seed will literally in this combination also restore more than 100% HP of Venusaur even when the targeted Pokemon has only 1hp left, each round, making Venusaur almost unbeatable without a Pokemon that has Psybeam and both a high special attack value and a high speed value like Mewtwo, Mew, or Gengar.
Detailed explanation of how the function behind it works:
for sake of reference: (N * max(1, int(0.0625*MaxHP)))
Note that this was fixed specifically in later editions of Pokemon Stadium and second generation editions, wherein Toxic / bad poisoning and leech seed will reset the counter to 0 each round, whereas the counter was ever increasing in the first generation.
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It's kinda funny that we have arrived at a generational knowledge transfer problem, in Pokemon. Google's results without specifically searching for "bug" or "glitch" will happily generate bullshit answers about this topic, always referring to newer editions and their online documentation.
Some results that were talking about this bug, just search for "pokemon red leech seed toxic bug":
Venusaur beats Charizard any time in the first edition. Given you use the correct attacks, of course, which are:
Toxin and Leech Seed, Growth and Razorleaf :)