off the top of my head, the big pink monsters from Prototype... i think theyre called Brutes. im not sure.
they were all over the place in the infected zones, and if you got within like a hundred feet of them, they would chase you to the ends of the friggin earth unless you killed them. just annoying
also, the little popcorn necromorphs from Dead Space that come out of fat necros. when encountering them, the player has only two choices: sacrifice a chunk of your health by allowing them to swarm you and then pressing A/X to shake them off, or sacrifice a chunk of your ammo trying to shoot all the little buggers. if you havent played the game, they are basically like Flood infection forms in Halo, except you have to shoot each of them individually. they dont explode and die together. also, its much harder to hit them than infection forms are.
and the purple crystal dudes from the Super Smash Brothers series. i really hated those guys. if i remember right, you had to fight like 99 of them or something, and they got tougher and tougher as you progressed.