People who hog the middle lane on the motorway. It's just plain lazy. Use the inside lane!
On single carriageway road, drivers who drive at an average speed of 30-40 mph, slowing down dramatically at each corner then speeding up on straights so that you can't overtake them. The A7 to Galashiels is notorious for this. If you're not confident enough to drive at a decent speed on these roads then you shouldn't be on them.