For me it's drivers that 10-20mph or more UNDER the designated speed limit in pretty much perfect conditions and no legitimate reason.
Share that one... even tonight... massive queue of traffic on a back road with 60mph limit, behind some twit who was doing 25mph for no apparent reason. There were no hazards, tight blind bends, or anything similar to justify it... just someone who felt like driving in such a way.
There was so much tightly bunched traffic behind this person... it wasn't even feasible to overtake with how far back I was... ended up delaying me about 20 minutes on that trip... watching the sat nav arrival time increase substantially... I'm used to seeing it reduce by a few minutes.
If that was me... I'd at least have the decency to pull over at a safe spot to let the faster drivers pass... but not this guy... wasn't even a blind old person or a tatty car... really no obvious reason for it at all.
If you can't drive at least close to the speed limit where it's safe to do so... you shouldn't be allowed to drive. Going that slowly can be just as dangerous as driving too quickly, because you become the hazard on the road.
Middle-lane hogs... not technically legal, as it could be enforced by a fine and even points... but it's rampant everywhere in this country (and Belgium notably). It significantly impacts the flow of traffic, while offering no viable benefit to the offending driver.
Slow drivers again... on a motorway... who, for some reason, are sitting at 50mph & causing the lorries to overtake... lorries should be the slowest vehicles on those roads & should never have a reason to overtake anything unless it's something that's a heavy load or similar that's unable or unsafe to maintain the same speed.
Similar vein to the first... those drivers pootling along at 40mph on a straight 60mph road... then they dare to flash at you when you perform a safe and legal overtake on an empty road with miles of visibility... what's their problem?
Tailgaters... on whatever road... it's unnecessary and unsafe... then you get a queue caused by one of the slow drivers & they follow so closely, it's not possible to progress by overtaking one car at a time because they're so close to the car in front, there's no space to pull back in again.
/rant