The practicality of having a flashlight so small that it can fit on a keychain has made the keychain flashlight one of the most widely used and popular flashlights ever made. Keychain lights range from 1-4000 lumens and can last as long as 70 hours.
Check out the
E02R to balance size and power.
If raw performance is what you are looking for, the
T4K is an industry leader in brightness.