>>28068655>Poison gen 1No.
Both Nidos(best non-starter mons around)
Tentacruel(best water mon behind Squirtle)
Gengar(stupidly good in general)
Victreebel(Stun Spore+Wrap+Razor Leaf+)
Venusaur(Sleep Powder+Razor Leaf or Toxic+Leech Seed)
Sabrina can be handled by facing her late when you are very well leveled and crippling her with status.
The only other threat is Blue's Alakazam, which can be handled by either Gengar or Tentacruel(both of whom can take a Psychic at level 51) statusing it so the others can wreck it.
Poison has some monstrously strong in-game mons in gen 1, and is also one of the types that easily gets past Brock and Misty.