The most annoying enemy in the game in my opinion is not a zombie at all. It is in fact, a plant;

The kernel-pult from i zombie endless.

These guys are just ANNOYING. Their butter attack will just eat your zombies alive, and they throw butter almost EVERYTIME. While you may think " well they dont always throw butter!" yeah, nearly every time it throws butter. And they even somtimes come in numbers of 2 or 3 along with powerful plants like split peas and repeaters just to piss you off even more. In fact i sometimes use up all my sun just to get rid of a lane with 3 of them, a split pea, and a snow pea. Just try it. TRY IT. If you havent tried it, YOU HAVE NO IDEA. I repeat, YOU HAVE NO IDEA.