Sewer Rats in Tales of Symphonia.
I'm talking about the little ones too, not the ones that are twice your size. I hate small enemies because their hard to hit and then combine that with the fact that they can poison you, I don't care how weak they are. As soon as the battle starts, I avoid every other enemy until the rats are dead.
EDIT: Oh yeah, and just because I have horrible luck, Enclave soldiers in Fallout 3. I wasn't even level 10 yet and one of those spawned (even though I don't think they're supposed to until Project Purity). He nearly killed me but thanks to a Stealth Boy I had I planted a couple of bottlecap mines around him. Unfortunately, he then saw me and the mines didn't kill him, but out of a stroke of luck, one shot from my shotgun from at least 20 feet away killed him.