For my purposes of having an Every-Day-Carry (EDC) flashlight that is always with me (ie on my key-ring) I carried an "Arc-AAA GS" (premium) flashlight for about ten-years.

After reading Doug Ritter's review of the Surefire Titan. I bought myself the "Titan A" model and have been EDC'ing it for almost a month. Like the ARC-AAA, the Titan-A carries well on my keychain, runs on a single AAA lithium battery (can also run on alkaline). The Titan-A has a 15-lumen output on its "normal" mode of operation (compared to 10-lumens for the ARC AAA). Additionally, the Titan-A has a "turbo-mode" -- which causes the light to put-out 125-lumens albeit with a very-short battery-life.

The one feature of the ARC-AAA which I like but is lacking in the Titan-A is that the ARC-AAA will automatically drop down its output to "moon-mode" when the battery is running low. yielding some hours of reduced additional light and a very noticeable warning before the light goes out completely. In contrast, the Titan's rated output of 15-lumens is available for 8.5 hours and then reportedly drops-off to nearly nothing without warning.

Thus, my perfect EDC keychain light would currently be an incarnation of the "Titan-A" with a "moon-mode", and would have the reflector coated with a long-lasting clear-coat glow-in-the-dark paint (ie. Strontium Aluminate with Europeum doping) so that when the light is turned-off, the reflector continues to glow. For examples of such paint see...

http://glow.glowinc.com/glow-in-the-dark-paint/new-category/

I know I'm probably sounding picky -- but when you ask for peoples' definition of the "perfect flashlight" -- you're bound to generate a flood of very-particular responses!