People whose itty-bitty clutch purses require a seat all to themselves.
People who carry on prolonged conversations on their cell phones in loud, carrying voices, blissfully ignorant of the glares being directed their way from numerous parties. (This is twice as bad if they make the call themselves.)
People who purposely sit on the OUTSIDE seat of a group of two seats so nobody can sit next to them...
...And then slide over to the inside seat the second the bus driver pulls away from the curb.
That guy in the very back seat listening to his iPod so loud the driver can make out the lyrics.
When high-schoolers discuss their illicit drug/alcohol habits for all the bus to hear.
The two teenyboppers in the seat right behind you who giggle nonstop at an obnoxiously high decibel, aggravating you to the point where you actually turn around and say to them, "Indoor voices please, ladies."
When the seat/person/animal next to you smells so strongly of spoiled milk/vomit/urine/pungent body odor that you can no longer breathe.