I have a PB Superflash Turbo on a rack mount that is available from PB and from a few other places. It is bright, and many have raved about it. I don't like the switch, but it is otherwise a good, solid light. It is relatively directional, but I'm okay with this for the distance it gets.

I also have a Princeton Tec swerve, which I like much better (switch, looks, feel), but I cannot find a rack-mount for it. I contacted P.T. about it and received no response, so I bought the Superflash. I use the Swerve clipped on to my trunk-bag on solid (decent side visibility, though the angle is usually off, and the extra brightness hits the ground closer than I hope cars approach me). The Superflash lives in the rack-mount in flash mode, and it adjusted to point its flashing correctly.