Streamlight is the best - Streamlight ProTac Rail Mount HL-X LED Long Gun Light

There is a reason why every flashlight that I own (with the exception of a couple cheapos) is a Streamlight. These are the toughest flashlights on earth and I have yet to have one fail me. I have Streamlight Stinger that I've used for almost 12 years and even though it is scratched and beat up it has never failed me and I've only had to replace the battery once.