I'm pretty chilled out behind the wheel, and very rarely get annoyed or gesture unless something is seriously out of order, but I have one massive bugbear, which stems from living in a very rural touristy area.

These are the drivers who drive at 25-30mph when they are enjoying the lovely scenery and looking at the rolling hills and the birds soaring overhead, but then speed up to 45-50mph when they get to urban areas like villages and towns, because getting to the next scenic bit is far more important than giving a shit about the people living in those areas, and think the place is purely for their enjoyment. That really fucks me off.