I don't hate any Pokemon, but there are some Pokemon I don't care for, and those are:
Gen I: Hypno. Now, I don't hate Hypo, but honestly? The creepypasta and that one creepy Pokedex entry about it have made me feel less about it over the years. Hypno's Lullaby has just made me feel pretty distasteful towards the Pokemon to some degree.
Gen II: I don't...really have a least favorite generation II Pokemon, but I guess if I had to pick I'm not a huge fan of Sunflora. I don't dislike it? But I had such a hard time using it in battle, even in that one Mystery Dungeon story where this Pokemon starred in. I guess, thinking about how hard it is to use in battle just makes it lower on the list for me.
Gen III: I also don't really have a least favorite among this group of Pokemon? But I guess Illumise and Volbeat would be the closest to me for "least favorite" for Generation III, I'm just.... incredibly indifferent towards these two Pokemon. I feel like they're just there for filler, or something.
Gen IV: Rypherior. I don't dislike Rypherior, but I just have never thought Rhyhorn really needed this evolution...? I suppose that's enough to consider that least favorite, right?
Gen V: There aren't really any I can think of, but I'm pretty indifferent to Amoongus, I guess. It just has never really grabbed my attention enough to want to ever try using it.
Gen VI: Binacle. I do think it's well-designed for a barnacle Pokemon, but... it looks really mad. I don't know, that's always made me feel unsettled about Binacle. Also, how does this Pokemon walk? It has no legs.
Gen VII: Gumshoos. I think the internet ruined this Pokemon for me pretty much right away. All the comparisons for a certain someone in politics just made this Pokemon really distasteful. I can't unsee it. But this only goes for regular Gumshoos. I'm okay with the shiny form, as it doesn't look like it who shall not be named.