As for who I hate seeing the most - playing as a class that doesn't have much burst DPS makes it very difficult for me to take on really tanky people. It just takes forever to kill them (if I can) so I tend to ignore people like Minniee, Citizenkane, Cyerr, Artocys, etc. Even if they go as DPS they have a knack for staying alive so it's best to ignore them and take down the squishy people first.
I also don't like running into Mccloud, once you realize he's targeting you by that point it's too late.
Wardens like Omnipotent and Slobodan are also a pain, they kill you too fast with their pets.