W501B with 5 mode Cree drop in. The drop in is worth it, flashing mode is bright. Find a narrow plastic bottle (ink dispenser, $1 stores, cut it, I got mine from an empty ink jet refill).
I use Planet Bike blinky mount moded with reflector mounts. It'll fit all kinds of flashlight and handlebar OD.
Diffuser beamshot, less spotty, more side diffusion than Planet Bike Superflash and host of other optic rear 1w blinkies.