Cyclists who obey NO rules whatsoever - go through red lights, even at major junctions, cycle down my one way road the wrong way so I could easily knock them off when I pull out of the side, swerve in front of you on pavements, have no helmet on, have no lights on or anything at night so they are basically invisible.
Pedestrians who just walk across side streets without looking that I am indicating and turning into the side street (meaning another car could go into the back of me, assuming I am able to turn off the road).
My opposite neighbours who are constantly living their loud lives outside their house - they are virtually never in it, and just shout at their kids all day and night.
People who assume I should be the one to change my line of walking with a phil & teds with two children, just to let them walk in a straight line.
And definitely the people who drive 40 constantly through country lanes, through the 30 mph villages and long stretches of 60mph, soooo annoying!
...Off the top of my head. I'm sure I rant about a lot more! And I'm loving Jamieandhismagictorch's link to that Catherine Tate sketch about not waking the baby - it is so understandably true!