I dislike the following pokemon from each gen:
1 - Mr Mime (it's weird and I can never get the hang of using the damn thing), Onix (its Attack is sooo disappointing)
2 - Igglybuff and Cleffa (pointless), Delibird
3 - Registeel (Steel attack fail), Beldum, Mawile (see Registeel), Feebas (aaaargh!)
4 - Electivire, Magmortar, Lickilicky, Rhyperior, Magnezone, Chimchar line, Togekiss, Happiny, Mime Jr., Rotom, Shinx line, Shaymin, Arceus
But I love Dunsparce : )