I agree with you on the antlions. When I was suposed to battle them with my car; no problem. When I was suposed to fight them whithout walking on sand: Big problem. When I goit the bugbait and walked to one of their nests in nova prospect, they would scare me to hell when they got out of it, and I accedentally shot every first one that came out with my shotgun.
Buddha… Unbeatable.
fast headcrabs/zombies and houndeyes
the first scared the shit out of me even before i met them face to …face whit those noises they make
houndeyes are waste of time and bullets
uh, and maybe the baby headcrabs from halflife1, one tiny white shit cant let you pass forward, and does imbalanced dmg
Not that i hate them, they are just annoying, i cant let myself be by hating something of the halflife series
I still hate the poison zombies the most. Hunched over, carrying a bunch of poison headcrabs AND THROWING THEM, making those noises? Fuggin’ creepy, man. 
I don’t hate them, per se…just creeps me out.
I think the “creature” I hate the most would have to be any creature who’s pathing causes them to get in the way of my movements. Usually, this is humans but sometimes it’s vortigaunts in HL2. But that’s really a programming issue, not a problem with the “creature” itself.
Also, alien controllers are sometimes annoying because they’re so hard to hit and, in the wide open parts of Xen, they can suddenly get really close to you and then move very far away in a short time span. I wish they were easier to hit. 
Scientist, because they’re cowards.
I hate “WET FLOOR” signs.
But those aren’t creatures, are they?
THINK AGAIN
How does this relate to Half-Life…?
Well, because Half-Life has lots of wet floor signs.