I truly hate Blissey - at least she's the only Pokémon for which I can find many *important* flaws. Yeah I know she's really useful when it comes to strategical battling, but even there, she's so overused...
I mean, there are not a lot of heal bellers, I know that, but there's at least Miltank...(which I actually do dislike, though not nearly as much as I loathe Blissey). Not that Bliss annoyed me as an 'un-defeatable' Pokémon, there are some quite good counters to her; the only fact of seeing her in nearly every team, with the same moveset everytime (oh, no, there are actually people making a difference between using her with Heal Bell or Aromatherapy... I don't.)... that's (one of the reasons that explain) why I stopped battling a year ago.
Then, Bliss is so... pink. I don't use it only to describe her color scheme - although I admit pastel-ish pink is my least favorite color. I mean, her attitude, nearly everything in the '(lack of, imo)character' they gave her is... pink. Like in 'we all live in this wonderful world where everybody is cheerful and caring'. I don't really loathe plushie-like thingies trying to become a virtual 'mother figure' and always making people happy just by smiling at them, but it gets a little boring at times. And she's 100% female! er, I did not intend to give my feminist views a place in my opinion on some Pokémon... well, it still kinda makes me angry at times to see every 100% female Pokémon being a mother figure or something like that, and every 100% male Pokémon being a warrior.
It's a binary point of view... and I think we should reserve binary thinking to computer science
There are some other Pokémon I dislike - Miltank, the Latis, Mightyena(it deserved better stats!), Suicune, Pichu, Cleffa(her crappy English name. Pii isn't that better actually, but oh, well.), Nidoran(both genders), Umbreon and Espeon (only when they are together. I mean, the dark, mysterious, 'so cool' Pokémon and its brilliant and so kind alter ego... too stereotypical maybe.), and some others, for various reasons I wouldn't like explaining as I don't like writing too much in one post. Especially in a post on a what-is-your-(favorite/least favorite)-(pokémon/type/attack/etc) subject. I still explain my opinion though, I am just not really comfortable with doing so for eight... nine Pokémon...