Drivers who constantly shuttle back and forth between 30-40 and 60+mph on a 60mph road, causing you to shuttle back and forth as you accelerate to a reasonable speed and stamp on the brake five seconds later, despite the fact that the nearest traffic in front that would cause them to change speed is at least one or two hundred yards ahead. But they don't shuttle at random, no; they go slow when you can't pass, and speed up when the opposite lane is clear...
I seem to be caught behind them all the time, these days.
And tractors. 20mph tractors. I hate the country.