As previously mentioned, the people who drive at the same constant speed regardless of what the speed-limit is for that road
And people who can not use the correct lane at a roundabout.... I have a few roundabouts near me where it is clearly sign-posted before the roundabout, then the lanes even have the signs painted in them and people still use the wrong lane for the exit they decide to take - If I have had a bad day and it is safe to do so, I will deliberately block them, as I drive a work lease car (so if somebody bumps my car and it isn't my fault I get things sorted without the hassle of dealing with insurances etc) and I know somebody who had exactly the same thing done to them where the car driver in the wrong lane actually hit them and got done for dangerous driving due to the fact that they were not following the rules of the road
As for motorbikes, I ride myself, and I can understand people who get frustrated with the minority that ride around like idiots, but I always acknowledge people who make the effort to move over, even when I am not looking to overtake as they realise that I would be able to pass in gaps that they would never manage in a car