Surefire Aviator (A2, I think). LEDs, bright, but with dimmer red light for reading maps, etc. You can get the dimmer light in red, white, green, blue and yellow. Red screws with night vision the least, hence my choice. Inside pocket of my suit coat each and every day (right side, so I can draw it with my non-gun hand, in case you're keeping score.)
Worth every penny.