Reliability: No matter how much or how little you pay for it, you only learn this when using the flashlight, after 6 months or so of carrying around and using, I start feeling a bit better abut it.
LEDs are more rugged than filament bulbs, that have a rather fragile filament inside the bulb.
Battery availability: Something similar to ammo, the more common the battery the better, and hopefully it will run with just one. Most stores will have AA and AAA. Others may be harder to find in some places.
Run time: How long can the flashlight run will usually be more important than how bright it is. A tunnel, cave, but also more mundane things such as blackout and moving around buildings when there’s no power.
Brightness: For more tactical applications or for signaling, good brightness is important. That’s why a flashlight with low and high mode , and strobe is so desirable.
On/Off mechanism: While twist on/off is more reliable and there’s less mechanical parts that can brake, its rather hard to turn on quickly using just one hand. For a flashlight that may be performing more tactical/self defense roles, a tail cap clicky is preferred.
Attachment type: Some of those little strings and cords like the one pictured in a flashlight in the post below, they will brake. Do yourself a favor and cut them away now so as not to trust the useless thing and end up loosing your flashlight. I replace the string with a metal paper clip, and wrap the middle with a bit of self soldering rubber. A metal swivel hook attaches it to the bag so I don’t lose it.
Finally, as great as ordinary flashlights are, they’ll never allow the freedom a headlamp would for working, when you need both hands available.
Just a few things to keep in mind when shopping around.