Picking a single light is easy for me, as my collection is heavily biased in a narrow niche anyways - 1xAA/14500/ efficient low-/sub- lumen EDC lights. I use my lights for long periods in dark conditions and mostly for close task work (eg, camping) and like using my night vision with low-/sub- lumens. The 1xAA format is by far the most versatile for me since: it's small enough for 24/7 carry, has all the performance of CR123/16340 (with CRAAs/14500s), is compatible with all my other camping/travel/emergency gadgets (AA Eneloops), can be found anywhere (AA alkalines), and in a pinch can scavenge common household cell (AA/AAA/9V).
I think I own, or have tried, just about everything in the niche and the versatility of my NW Quark Pro AA XML (QPA-X) is the one that cannot be beat for my purposes.
- 350 lumen max, 200 hr moonlight (yet bright enough to suffice as my only mode)
- floody for close task work, yet out throws most AA/XML lights, ie good balance
- tough (for an Asian light) with US-style warranty, HQ and CS location
- most field/user serviceable/repairable, and of course tons of Lego options.
- fully supports CRAA/14500s and in a pinch, can easily use anything from a CR123 to 18650 with just the head and a piece of MacGyver tinfoil.
- my favorite peudo-feature, however, is its momentary max function from any preset lower mode that works just like car high beam flashers - no need to long click/short click/wait/hurry/stand on one foot and hop/etc.