Heh, yeah, that's the center of the crazy flashlight nerds. For keychain stuff it doesn't matter much, anything with a few buttons, AAA, or AA will do the trick.

If you want to see across parks, tops of distant trees, flag down helicopters, etc then get an 18650 based light.

Fenix is a good place to start, but there's many companies that made good lights. Even $50-$60 buys a hell of a light .

+1 on Fenix, Eagletac is another great reasonably priced brand that I've found held up well over the last few years.