https://calc.pokemonshowdown.com/?gen=1Charizard 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.)
Blastoise Surf vs. Charizard: 205-242 (57.1 - 67.4%) -- guaranteed 2HKO
+2 (swords dance) Charizard Hyper Beam vs. Blastoise: 194-228 (53.7 - 63.1%) -- guaranteed 2HKO
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
QED