Black headcrabs from HL2 and the spiders / zombies from Dark Messiah of Might & Magic. All three have an ability to drop your health to 1 instantly, which, imho, is the most frustrating implementation of poison I've ever seen.
HL2 at least used that with some moderation, so it was okay, but DMoMM sometimes got plain frustrating because of that.
Any enemy with an overly potent stun ability in any genre. Nothing is more infuriating than being beaten up by some weaklings because some Medusa / spellcaster / whatnot baddie stunned you for 10 seconds.
Oh, and I absolutely love zombies, normally, except for DMoMM