Must have:
LED
Remember last brightness setting
Low output around 10 lumens (this will do for 90% of the daily use)
LONG run time on low
About the size of a fat Maglite
Water resistant (ideally 1m or 2m rating)

NO BUTTON CELLS

Nice to have:
Able to use rechargable Li-ion standard size batteries

Dream light
Able to charge battery with USB or magnetic charge cable
Able to charge other devices with USB out

I currently EDC a Fenix PD35. It checks all the boxes on the must and nice to have lists.