I think it’s a tie between Jynx and Mr. Mime. I really shouldn’t be one to bash a Pokémon due to its ugliness, but I personally just find them really ugly and gross with no redeeming features outside of their typings and pre-evolutions.
In my childhood I didn't dislike anything, but when I got Emerald Zubat really got in my nerves because of how it was in every cave. Then in Gen V I choose Hydreigon because it was hideous and disappointing, then Aromatisse because it was even more hideous and disappointing and then Alolan Golem because it was hideous and disappointing to a new degree. Gen 8 has a few disappointing creatures but thankfully not to the degree of those.
Durant, and also Kricketot and Kricketune
more due to the species its based on - those two insects (ants and cockroaches) seem to ruin everything I have saved, somehow find ways inside even when I have everything very sealed, an hour later they are inside, I have to waste food beaches of them
they are so annoying to get rid off
every time I see the Pokemon, they remind me of the pests
from the Pokemon I given a try before - Gardevoir and Gallade
like the typing, really dislike the designs
I don't really have a least favorite but I do vent a lot about the Pokemon, Eevee.
God, it's so overrated, everyone loves the damn fox... dog... fluff... thing. It's got mountains of fanart, constant pandering from Game Freak and Players ever since LGPE came out. The constant supply of Eevee creepypastas have stacked above the height of Lavender Town Tower, and those weird art of how the species is 'sick of evolving' when it's one and only purpose is to evolve into one of multiple types. God, my salt for this overrated fox-thing has boiled so much that I even dislike it's evolutions due to the sheer toxicity of it's entire fanbase. Overall, I do like Eevee. It's just that it's incredibly overrated.
My absolute least favorite Pokemon is Bellossom. The rest of the Oddish line consists of my absolute favorite Pokemon, so it's a bit of a shame. It loses its poison type and becomes monotype grass, which I am reeeeeally not a fan of. Its design also doesn't feel like a smooth progression from Gloom like Vileplume does. Bellossom was also in USUM as an NPC, even though Oddish wasn't even catchable in those games, so I'm also super annoyed about that because Oddish is my favorite and I'm petty.
I also hate Thundurus, Landorus and whatever the last one was, because I think they're super hideous, and way too palette-swappy. The therian forms are a lot better, and I wish they looked like that from the get-go.
Then there's the Pokemon I dislike because I hate fighting them; like Lapras, Skuntank, and Bronzong.
I have a few that I have an absolute hatred for: Trapinch, Glalie, Dugtrio, Zubat, Graveler, Unfezant and Liepard. They all have one thing in common: Stopping the battle from becoming a smooth and fast experience when against the AI.
But if not counting towards battle experiences and simply in general, I will have to say Charizard, that stupid lizard got way too much attention, 90% of that attention wasn't even deserved in the first place. It's starting to feel like Charizard is not even a starter Pokemon at this point, which is a telling sign of too much favoritism, and you know how I absolutely hate favoritism in any video-game that has no lore-related reason to be there.
I have like 3 least favorite Pokemon, but the main offender if Mewtwo. I hate how absolutely overrated it is, especially in the anime where it's gotten three movies and two special episodes (1 of which is like two hours long ffs). I never saw the appeal of Mewtwo even back in the Gen 1 days; to me the Frieza/cat fusion look is freaking hideous.
Scyther. I never liked this thing. As a kid the idea of a giant bug terrified me, but I lost my fear of Scyther when I got older and back into Pokemon.
For the heck of it, I decided to do a Nuzlocke in SoulSilver. It was my first nuzlocke ever, I had only recently gotten back into Pokemon, and it was going well. Then along came Bugsy with his Level 17 Scyther.
My Pidgey, dead. My Rattata, dead. My Mareep, dead. My Ekans, dead. My Sentret actually somehow managed to take the Scyther out after I lost over half my team, but then died to poison from Metapod, when Scyther had Switched out. I barely managed to make it out of that fight with only my Chikorita standing, who evolved into a Bayleef after that. But it didn't matter, I quit that nuzlocke. Scyther glued itself as a horrible Pokemon in my mind, and I was done with it.
It certainly did not help when my ex (while we were dating), heard of my hatred and for giggles bred a competitive Scyther who he would use against me in our friendly battles. I hated that thing so much.
So that's why I hate Scyther.
Honorable mentions go to the Regis because of the nightmares of my childhood they fueled with their horrifying anime cries from the Mystery of Mew movie. Especially Regice.