I would say illegals (no drivers license or insurance, can't read road signs quickly, too short, and started driving too late), and non native born Asians (too short, can't read road signs quickly, started driving too late) are the worst offenders.
In general if you come from a country that relies heavily on public transportation, and didn't even spend you childhood in a passengers seat, you will never be anything but an indecisive, awkward, dangerous driver.
But I am just a bigot.
I grew up in NYC and live in the LA area now. As to the Asian drivers, I would say that some drive poorly because they learned late in life, or at very low speeds because of the crowding. Most of their kids don't have that problem. I would rather drive in a Chinese or Vietnamese area than a Korean one. They tend to be indecisive, and do crazy maneuvers(a four way turn in the middle of busy and active intersections), regardless of heavy, fast moving traffic. Another stunt is "I'm lost!" and they then come to a dead stop on the freeway. they are clueless. I believe them to be the worst drivers I have ever seen anywhere in the world.
I would say teenagers are the worst, especially when they have a cell phone stuck to their ears.
I watched a girl run 4 consecutive stop signs while yacking on the phone. Thankfully it was not a major highway with heavy traffic.