Vehicles (often illegally) modified to be louder than normal solely to be annoying is probably my top one.

I absolutely hate that one. Why do you need to remove the thing that makes your motor make less noise? You're in the middle of the city! The amount of absolute lack of brain cells needed to do that is astounding to me. They think they're cool! They think people look at them and think "wow what a cool guy!" They do not understand people think they're losers, it's wild.