Generation I had its fair share of unpopular Pokemon. However, not too many were actively disliked, so the ones that were hated most certainly deserved the disdain.
For reference, this article isn't about inherently bad Pokemon. This is about Pokemon that aren't fun to fight against as doing so can feel like a chore.
This also isn't about the Pokemon themselves but how they were portrayed in Generation I.
Note: This article is subjective and reflects the opinion of the writer.
What are the 3 most disliked Poison Pokemon from Kanto?
#3 - Tentacool
Tentacool is the only Pokemon in Generation I that can be found while surfing on the parts of the ocean that are accessible in southern Kanto.
Unless the player is diligent about keeping up repel (which is a chore to do in Generation I with all of its relatively slow menuing and limited item space), they can expect to hear more than their fair share of Tentacool's cries as they make their way to Cinnabar Island, the Seafoam Isles or around to Fuschia City.
By the twentieth or thirtieth escape from battle with the same jellyfish Pokemon, many players will be severely irked with Tentacool.
#2 - Muk
Koga's sudden-death Explosion Weezing is annoying, but the Fuschia City gym leader's Muk is far worse.
Koga uses a filthy strategy of spamming Minimize until Muk is near impossible to hit. It then uses Disable to reduce the chances of the player even being able to use a super-effective move. The Pokemon uses Poison Gas to drain the opponent's HP and Sludge Bomb for damage.
Even when the player does manage to land a hit, Muk is a special wall. Only an Earthquake from a solid physical attacker will OHKO the grimy Pokemon. While options like Aerial Ace exist, using them is still just a damage race to see if the player will be freed from Muk's torment eventually.
#1 - Zubat
Everyone has their horror stories of traveling through Mt. Moon or Rock Tunnel and getting swarmed by a never-ending torrent of these blind bats.
Unlike Tentacool, Zubat is not a guaranteed encounter. However, Zubat is not only fast enough to outspeed many Pokemon (even ones higher leveled than it), but it can cause some serious annoyance if the player tries to escape by KO'ing it.
With Supersonic (which always hits for Zubat but never for the player), confusion was basically guaranteed. Oftentimes, players would end up more ragged from dealing with the accursed bats than the random NPC battles spread throughout the caves.