Other bikes that their rider doesn't let me know they are coming up behind and passing. That's annoying both when I'm cycling and walking.
If it has to be a motor vehicle, then the ones that stop for me at crossings. They have the right of way until I actually am in the crossing. I'd rather they just go on. Especially when they are the only vehicle near by on a four lane road. Some drivers actually get mad at me for refusing their generosity. Some are so insistent about waiting for me cross that I have to point out to them that I have a stop sign and they only have a yield sign.
The only time I'll cross for a stopped vehicle is if it is a one lane road and the traffic in front of them is stopped.