For me, it's Pikachu, Charizard, Lucario and Mewtwo. The "trifecta" of what I dislike about Pokemon. They are overused in everything despite people actually liking them. I don't hate these Pokemon. I just don't care for the fact they get first pick over everything even if they aren't right for it. How TF does Charizard get picked over Pidgeot for the PokeRide feature, a Pokemon designed to fly twice the speed of sound and can effortlessly fly through storms and hurricanes, some islands are prone to? Oh, because it's Charizard. How does a Pikachu in cosplay get a lucha spot in Pokken Tournament over Hawlucha, a Pokemon designed for that role? Oh, because it's Pikachu.
I liked Mewtwo better when it wasn't everywhere and being catered to. I liked it better when it was just Mewtwo, you know? As for Lucario, it was just everywhere for no reason at all. Before it had the chance to be popular, its face was everywhere. I guess I don't really like it because of furries. Now, I don't really hate these Pokemon. I really love Charmander but I'd like to buy a Pidgey plush sometimes and I don't have that option because of those four. They are overshadowing all the other Pokemon.
But in all fairness it has nothing to do with Pidgeot, just an unbiased view right?
In all seriousness you're not wrong. I mean Pikachu is the mascot, so that's that but it can get very overwhelming sometimes with all they release for Pikachu. But there is no real reason to favor Charizard, Mewtwo or Lucario. I mean if I got to choose a flying ride Pokémon Charizard would not be my first pick (nor would Pidgeot though). As for Pikachu over Hawlucha in Pokken, I heard there was a fight and Hawlucha lost because of his weakness (electric beats flying), ah well fair is fair j/k. No it probably goes back to the mascot thing. Though personally I dislike Hawlucha so...
There are lost of Pokémon I technically dislike but nothing really stands out above the rest. Mewtwo does come as one of them, for really no particular reason that I can name, I just don't care for it that much. I would say however it was always everywhere, from the final boss (twice) in Colloseum to even being the biggest challenge in R/B/Y, it just wasn't that obvious at the start.
Really in all honesty I've always had a small dislike of normal, poison, bug and fighting types. For me they have to have some good second typing to make it worth using them.
Joltik, Pumpkaboo, Lanturn.
Why?
N i g h t m a r c h
For me, it's Sunflora.