My most used light is an internet acquired Fenix LD01, single AAA battery, three levels of light. Worn on the keychain, it is right there when I need it, the most important consideration. The next most important feature is that it turns on when I need it. A nice simple twisty with enough friction so as to not screw on or off inadvertently, the LD01 measures up here. Everything else is just details.

I load mine with a primary lithium battery which lasts at least a year in my normal usage pattern. I routinely carry a spare battery, meaning I could hike all night long for at least a couple of nights, using nothing but this light.

More and more, I prefer electronic items which can interchange batteries and which can chow down on alkalines, rechargables, or lithiums, whichever is at hand.