Cutters - Dead Nation
Especially once you start playing on the harder difficulties. I shot those damn things with a rocket launcher 10 times and they still come running at you.
Zombies - Ocarina of Time
It was always frustrating having these things set their gaze on you and scream. Thus freezing you and making it impossible to do anything while they close in and chomp on your head.
Shredder - TMNT Aracde game
He was very annoying, and it just got more and more annoying having to go through ten dollars in quarters just to beat his ass.
Tasers - Pay Day: The Heist
One shot from those guys and you're pretty much done for. You just stand there getting shocked unable to move until you either die or someone on your team takes him down. Very very annoying.
Bolters - Red Dead Redemption: Undead Nightmanre
I love all the zombies in red dead, except for these guys. They run on all fours and are incredibly fast and tough to kill.
Hammer Bros - Super Mario Bros
Or more specificly, the hammer bros in world 8-3. I think I still get anxiety from those guys on that level.
Mike Tyson - Punch Out
Apparently the guy is invincible. I remember having a subscription to Nintendo Power and in 1997 they had a ten year anniversary section for Mike Tyson's Punch Out. In the article it stated that back in the late 80's, only four people had won against him and those were all by decision. As time went on, people proved he could be beaten however I'm not sure if those people were cheating and modded the game. Regardless he's my #1 most annoying and frustrating. Because what's more annoying than an enemy you just can't defeat?