I voted 'elderly', but I'd narrow that down more and say elderly women. And if it's an elderly Asian-Indian woman, might as well just pull to the side and take a nap. Same with any car with more than just the driver; two or more elderly women in a car and you're begging for road rage.
(The ongoing joke around here is that it's mandatory you must buy a Buick LeSabre at the age of 65).
Second place, at least here in NJ has to be hispanic teens. They're notorious for weaving, running lights and stop signs and using turn lanes to pass; they lean so far over, they look as if they're driving from the passenger side. They don't drive, they cruise and race and God help anyone in their way. Add cell phones to that combo, most times. At the stop light on my own street, when our side gets the green, we all know we still have to look both ways before going; even then, I've come extremely close to T-boning these kids multiple times.
The laws in NJ state that for the first year of driving, a teen can't drive alone after midnight-I'm not so sure this is a great idea as a car full of teenagers is a distraction to the driver. Add the blaring music, a couple of cell phones and we're talking calamity in the making(there's been a rash of fatal accidents involving high school kids here lately-perhaps a result of these laws?)