There are so many I can't even rank 'em.
#1 has to be IDIOT DRIVERS. Y'know, the folks that cut in front of you without looking b/c they're too busy yakking on their f***ing CELL PHONE. Now, I own a cell phone, but the only time you'll catch me using it in the car is if (a) I'm pulled over or (b) I'm calling 911.
#2 has to be idiots without brake lights. In Connecticut, safety inspections aren't even the law. More than once, I nearly rear ended some a$$hole who stopped short in front of me, because he didn't have any brake lights.
#3: Then there are the people who can't be bothered flushing the toilet. Ewww.

I have some of those at work... ewww...
I will add some more when I'm more awake...