Smoochum.
Easily forgetable because it isn't anything special, it evolves into Jynx which isn't any special either, ontop of that the only way to get it was breeding off Jynx and it has no use after it whatsoever.
Though, you could catch it in Gen IV only in swarms and in Platinum, quite rare in the Snowpoint Temple.
I'm not saying it's the worst pokemon, but also not superficially throwing in Bug pokemon (which are common, and easily forgetable aswell) for a measure, it's just that i often forget that Jynx had a pre-evolution which is Smoochum.
From all generations, i think Gen IV was the most nightmare-ish for me as it featured 2 of my "i seriously don't like them, seriously."-pokemon.
Most annoying pokemon though is Bronzor... i don't know, it just... i don't like it :/ (Bronzong is okay though)
second most annoying is Kricketune, years and years and i'm still disturbed with my imagination why a bug would have a ballgag as a mouth.
Female version is worse though, moustached bug with a ballgag. :'c