For me, I gotta say Mirelurks(they look awesome though), Mirelurk Kings, Deathclaws(always tend to appear when im near death or low on ammo), and Boatflys.
These have annoyed me the most. :banghead:

Anything that takes more than a clip to kill.

Overlords and Deathclaws.

Rad scorpions. 10 XP for killing it. Thats the same as a [censored] raider, and a raider only takes a single shot to the head.

Damn bloatflies annoy me the most. :spotted owl:

reavers

Simply annoying? I'd say bloatflies. There I'd be dittybopping along listening to Three Dawg for the company when WHAP! "WTF?" sez I, looking around and looking around. Sometimes it takes three rotations of looking around before I finally find the damn thing and I wish all 'flies would die - I hate wasting ammo on them. :banghead: :facepalm:

regular mirelurks-its hard to hit the small squishy head. and it keeps headbutting me grrrr, don't get me started on the hunters.....

Never came across any death claws or overlords...and king mirelurks.

On pure annoying factor, Bloatflies. They're a waste of ammo. And for easily killing you, it has to be the reavers. I avoided exploring Point Lookout outside of the main quest because of those damn things destroying my armor with their radioactive guts and nearly killing me. A sneak attack barely does a dent in their health.

As for "annoying" .. I'd say the pesky little "Bloat Flys" .. especially when they start weaving back and forth.

However, the enemies I just plane old hate the most are the "DeathClaws", mainly because of the damage they can do and how much damage they can take. Just imagine 3 deathclaws coming at you at one time .. I went to Old Olney to get my Medic Armor and saw two right off the bat, I tried sneaking by them, one spotted me and I opened fire on his butt, it brought it to the attention of two more.. woohoo? I had a Gatlin Laser so I managed to kill them all, but if I would have been stuck with another weapon that does less damage or fires slower.. I'm sure I would have died.

Yes the deathclaws and the reavers are annoying, really annoying
also those inbred hill billies in point lookout kinda creeped me out

deathclaws deal some serious damage dude
overlords...dear god extremley annoying to kill damn super mutants
mirelurk kings ehhhh they dont bother me much easy to kill

Any animal.

Fawkes! i died once becouse he opened fire on me!

The freaking hillbillies in Point Lookout aggravated me to no end. Something like a Reaver having a boatload of health, I can understand. But an armorless, deformed, inbred hillbilly? C'mon! No way in hell they could soak up two clips of plasma fire!

Wait, Bloatflies? I can walk up to one and kill it, melee! How are they hard to kill!? And, I purposely went to the Deathclaw sanctuary becuase I was hunting them and I thought there were Deathclaw gauntlet schematics in there, but hey, Vengeance was a nice replacement.
And I probably going to regret saying this, but:
I'm playing on Very Easy
**Puts on Hellfire Armor for +25% Flaming Resistance.** :flamethrower:

IMO, those are the fun things to kill.
The most annoying enemies in the game are the bloatflys or the radroaches.

those things make bullet sponges look weak. once i ran into point lookout with a nice gatling gun, had to leave early cause i burned up all my ammo.

but my most hated monster? albinos, i just recently ran into my first one and that thing made super mutants look pretty. plus the poison doesnt help.

Albino Radscorpions. They are moblile, take little damage, and have no weak point.

Albino rads!

Feral Ghoul Reavers by far - tough, ugly, and hard to kill

Also, the enclave hellfire soldiers(I think that;s it's name) Anyway, I just hate them, really annoying.

you to?!

Mirelurks are super aggravating and only make, from what i can tell, a sound when they attack you.
Also for some reason Talon company attacks me in weird spots like the arlington library, and in front of the citadel