Human - Gotta be the Legion. Not for the actual attacking, but for the way that wherever they pop up, there's always a merchant nearby who starts taking potshots at them with a .357 revolver, and then gets their arse handed to them. I dunno what it is, but I hate seeing Wastelanders getting taken down by Legion

Creatures - either Ants or Cazadors. I think everybody hates the Cazador swarm, but Ants just about win it for me because of the sheer number of them in the game. It's like the developers were like "Hey, we gotta big open desert space here, what are we gonna fill it with?" ANTS!

Cazadors fucking bastards fly around spitting at me then the after effect sounds of being poisoned freaking annoying as fuck

Cazadores of course, fast moving, poison tend to hit you in groups. Nightstalkers would be a close second for me though.

Wow. I know that Cazadores are pretty damn hated by the Fallout community, but damn. I hate Cazadores as well, but there is just something about Radscorpion's appearances that always scared me. The way they move, the way they look with their beedy eyed faces. They're just...ugh. They are also real life creatures, so that sucks too. For instance, when I had Fallout New Vegas for the XBOX 360, I was doing the Brotherhood questline. I was at the part where I had to go to the Radscorpion Gulch. Let me tell you, I pissed my pants doing that quest. Also, when I got that perk that gives you all the locations in the map for you to explore, I decided to make a goal with my level 30 character and discover all the locations in the game. During my goal, I decided to go to Camp Guardian. Little did I know that it was infested with Radscorpions. My god, I had sneak my way through Camp Guardian.

For me It'd be Albino Radscorpions; Those things laugh at you when you shoot them. And for humans probably legionary assassins or Fiends.

The scorpions don't bother me because my hunter's pet in WoW is a scorpid (mind you, scorpids are a lot less creepy than scorpions, but still basically the same thing).

Radroaches on the other hand... ugh. I detest roaches in real life, so I kill radroaches every time I come across them. I even made a quest to kill tons of them, if that tells you anything.

Anything to do with the Legion. I hate 'em all.

P.S. inb4Cazadors

I would have to go with cazadors as well, they make it feel like a chore to shoot at them, and making you pray for it to be the last cazador you ever come across.

I had a bad encounter with a Nightkin. I was investigating this dark sewer-like place and there were regular super-mutants around so I was be all careful, but then I had to go and turn around. I take a couple of steps and BOOM uncloaked Super Sledge to the face...I needed new pants. Since then I hate fighting nightkin.

PS.
Cazadors also suck.

PSS.
So do Deathclaws

cazadors! as most of us have said they are stupidly tough, fast and they can poison you.

I also hate Bullet Sponge's unless there is a lore/logical reason why they are so tough, thats why I don't mind super mutant behemoth's.

I also hate Hit squads mostly because they always seem to attack you in the same area's all the time and they always seem to attack after a set amount of time.

Another hated of mine would be the damn Legion Assassins, I seriously run into them EVERYWHERE. Even one time when i was doing a quest in Freeside they came through the gate and started blasting me lol.

Another enemy that really grinds my gears(lol) are the Dead Money hostiles. (Ghost harvester,seeker, trapper etc.) they get quite uncomfortable at times, and same as cazadors, I get tired of them.

the scorpions they freak me out and i have arachnophobia

I hate any non-humanoid enemy...they're usually just annoyances.

As for emotional dislike - this is raiders and slavers. During gameplay - Deathclaws, of course (talking about Fallout 3, I didn't play NV much yet). x)

Feral Ghouls. Far too many times have I investigated a metro or abandoned building and our of nowhere get hit before hearing their shriek. Gets the jump outta me every time.

Mine are definitely Deathclaws. They're so silent when they run, I'm always getting attacked from behind by them which usually results in instant death. Always makes me remember to install CASM though haha.

Cazadors. I don't mind any difficult enemies like Legion Assassins or Deathclaws, I actually like the challenge they give you, it feels really good when you survive a ambush by enemies like that.

But Cazadors, that SPLOOSH sound is just unacceptable. I don't know how they managed to make it so annoying, it's just absolutely horrifying. And Geckos, they really make me angry for some reason. Especially Fire Geckos, you can't see anything when they do their flame breath crap.

Definitely cazadors, those pesky little flying bugs...
They are the sole reason I fought them in VATS...

I'll be honest here, Robo Scorpions. They are so god damn power full, exploding on death, a huge amount of DR and a fair amount of DT. They are a pain to take down in numbers too. So take my advice and don't get surrounded, or trapped by them.