My principal requirements are that it turn on and off reliably (no turning on when inside the pack), have a lanyard attachment, accept standard (AA or AAA) lithium, alkaline, or rechargeable batteries, enough run time to get me through a night of hiking (say ten hours), be reasonably water resistant, and fit on my head or helmet.

A headlamp is critical in any situation where you need use of your hands (caving, nocturnal rock climbing, heavy duty first aid) and can easily be carried in other situations.

A former requirement was that it house a spare bulb right with the light, but that is no longer pertinent in the LED era. The only incans I have are museum pieces, holdovers from the 20th century.

I have one other museum piece(s), a carbide lamp - fits all of the above criteria (except water resistance) and was a surefire fire starter and hand warmer to boot! Carbide is just too hard to obtain these days, the technology is obsolete, and the lamps do not do well around automobile and plane wrecks.

Currently I usually EDC two lights, a Gerber Ultra Infinity, and a Fenix LD 01 on my keyring. A Princeton EOS (headlamp) is never far away. I also EDC spare lithium batteries for the first two.