Mostly because I'm working through a Solo Legendary Reach campaign, grunts are gonna take my vote on this one. I don't care what kind of grunt he is. He. Is. A. Prick.
Plasma Pistol Prick: Overcharge + like 4 shots from anyone else with a PP, and yo' dead. Also, there's usually about 5,000 of these guys at once.
Fuel Rod Prick: 1 (fuckin 1!) shot and you're doomed. Worst when you don't see them amongst all the other 4,999 pricks.
Needler Pricks: Needler needles home in on you, and take forever to reach you (this means that right as your shield should recharge, the damn needle will hit you and keep it from doing so).
Suicide Prick: Not bad once you learn to kill all the grunts before taking on an Elite, but still. Prick.
OH, and now the grunts aren't as much comic relief due to the choice to not translate Covenant battle chat.
Anyway, guess I just needed to vent. This Legendary thing will kill me yet.