The riddler from Arkham City. He trolls you so bad when you fail a challenge it drives me insane. I want to take his staff and shove it so far down his throat that I could pull it out the another end.
Also I hate most enemies from Gears of War 2 horde mode. At the moment, I'm thinking I hate Boomers, Sires and Grenadier Elites the most:
-Boomers because sometimes a Boomshot round will kill me while I'm holding a shield, even though it appears to just hit the shield. Also for the times they hit crazy shots when usually they're nowhere close.
-Sires because they are constantly knocking my shields over, and bugging me when I'm trying to fight something else.
-Grenadiers because of their random grenades. I might know a grenade is incoming, but unless I see it I don't know whether to move or not, since they miss a lot. And I never seem to be able to shield myself correctly, even though I swear I'm facing the grenade.
Other enemies can also be annoying, like Therons, Kantuses and Maulers, but at least you get cool stuff from killing them.