The 4SEVENS Quark AA Tactical Flashlight is my EDC (Every Day Carry) light. Honestly, I use this thing all the time. It’s about the length of a roll of quarters and sits length-wise at the bottom of my left front pocket.
It is an LED light and has a programable head with seven output modes: Moonlight , Low, Medium, High, Max, Strobe, Beacon (You get to pick two). I usually run the light on high (which is enough for most tasks) to save a little on battery life. My second setting is Max mode. The “Tactical” part of this light refers to the tail cap which protrudes from the back slightly and allows for momentary on/off along with the full click on/off.
It has a durable T-6061 Aircraft-grade Aluminum Body which is holding up well to being dropped on the floor in my pants daily and more than a few trips through the wash.
The primary reason I chose this light over, say a Surefire is that it takes one very common AA battery. You can find a replacement battery for this anywhere and I suspect it will be easier to find one of them rather than trying to find a CR123 or some other exotic battery after the SHTF. I also keep a spare on my keys in a waterproof case which has come in very handy.