As a driver:
As a cyclist:
People who think it's funny to shout abuse at you when driving past. Though when the stop at the lights a few hundred metres up the road their door mirrors seem to fall off.
Some of the most aggressive road users I've seen (in London) have been cyclists. Shouting abuse to all and sundry. Usually when they've steamed it through yet another red light and the wrong way up a one-way street