UE Megaboom.

Requires a wire bottle cage as it's slightly oversized. But it's worth it as I roll by like a Camaro with a whole rack of amps and subwoofers in the back.

People don't like it. I don't care. I find that I get far fewer cars pulling in front of me, right hooking me, and fewer pedestrians wandering in front of me midblock without even trying to look first.

Plus, I get to listen to my music all the way to work.