My current light...
My daily carry light for the last four years has been a Fenix PD35. This has been a totally awesome light. My eyes are going bad so I use it a lot. I use it for work, outdoors, hiking, working on vehicles, everything and everywhere. Great solid light. There are five brightness settings and a strobe. I usually run it on the lowest setting but sometimes on number two when needed.
To power this light I use a Samsung 18650 unprotected cell. This is a flat top industrial cell (which the light accepts since it has a spring on each end of the battery chamber). So as not to overwork the cell, I never run it down too low - and I don't charge it fully up. Even so, it lasts a LONG time in the light.
I believe the light has circuitry to prevent over discharge of the cell but I've never hit that point. I charge the cell on a "smart" charger that is *supposed* to be safe for 18650s. BUT, I never leave it charging unattended. I'm still on my first cell (haven't worn it out yet).