Dangerous drivers. People who, when waiting in the special turn lane at an intersection to turn across it, decide at the last minute they don't want to turn and slam on the accelerator to move ahead of the first car waiting to go straight. I almost rear-ended someone who did that to me.
Raging drivers. That was me after the above incident and I chased the car with every intent of pushing it off the road and going after the driver. Luckily I came to my senses in time before I did anything so stupid.
Drunk drivers.
Aggressive drivers. You are safely several car lengths ahead of a car and put on your signal to turn into the lane they are following. For whatever screwed up reason, they slam on the accelerator as you start to make your lane switch so that you have to quickly go back into the lane you came from to avoid a collision.
Drivers who, due to a combination of age and declining mental facilities, shouldn't be on the road. They do dangerous stuff like drive half the speed limit , crawl out into an intersection from a turn lane on a green light but stop completely in the middle of the intersection when it goes yellow, or turn from a shop or side street into oncoming traffic.
On the less bad but annoying side - drivers who signal to turn into a cross street but do not move to the turn lane or do so only halfway, forcing through traffic to slow down to accommodate them.
Passengers:
Passengers who needle, annoy, provoke (physically, verbally or otherwise) drivers. Or who are thoughtless with their words and the impact they might have on the driver at the time. Imagine crossing a bridge and at that moment telling the driver he just lost his job and cannot support his family or meet his financial obligations anymore.