My main tail light is on the helmet (Knog V4 Blinder, 4 LEDs and very bright). I also have a couple coin-cell blinkies on the saddlebag flap, as backup. Oh, I also have some Trax bar-end lights, but I don't think they are all that effective as tail lights.

On the rear, reflective is as important as lighting, because the car's headlights are more powerful than bike tail lights. I save most of my lighting efforts for the front.