oh, such a toughy, but i have to go with either jackals from the halo series, suicide grunts in the same series, peach, the demon animals from the legend of zelda, or toad
Jackals: their shields can be very annoying, and they keep you occupied while elites or something kills you
Suicide Grunts: if you cant see them coming, or if you run into them in a corner... BOOM!
Peach: she uses toad as a freakin shield in super smash bros!
Demon Animals: they just wont die! no matter what you do!
Toad: obvious reasons, plus there is some speculation that he works for bowser, he keeps directing you to wrong castles, and he is the only one to survive besides lakitu in super mario 64, coincidence? i think not!