Little black flies. Mosquitos can be swatted, or kept away with bug repellent. Those little black flies are too small and quick to swat effectively and ignore bug spray unless you're soaked in it. They also leave much bigger/itchier welts and tend to bite around the eyes and mouth. Truly they are Satan's insect.
People who dont feel empathy for animals or event other people, mainly animals. There's not a thing that will make me want to bash your head towards the concrete than knowing you hurt an animal
In addition to all others mentioned so far I'm going to add horse flies. They are stupid but very persistent and don't just want your blood, they want to carve off a piece of you and fly away with it.