I could never see the need for carrying a light with me..... until I started doing so and realised it (along with my knife) puts the "ED" in EDC from a daily usability viewpoint. I've used mine countless times in bright sunlight to peer into small dark holes (for JS4's sake (!) I need to qualify that by adding that checking that holes in machined parts are drilled deep enough and properly threaded makes up a far more significant part of my job than I ever thought it would...)
My EDC light used to be a S&W M&P branded PowerTech light. Single-cell CR123, 86 Lumens and I'll freely admit I bought it for the branding. It turned out to be a really nice light, fits comfortably in your hand (it's an M&P after all!), and small enough for comfy pocket carry. It's major flaw is it's "trigger" (and here I can hear some of you say "it's an M&P after all"...!) The button function is all back-asswards: no momentary on, you gotta click it on, and then you can press it slightly for momentary off! Probably designed by an industrial marketing consultant... I could still live with that (just needed some muscle memory) but the fatal flaw is that the "on-click" is really easy, and when sitting the button pushed against my belt, which led to a few occasions of "hot glowing pocket". Apart from the obvious discomfort and embarrassment (when you walk into company and you get those "what's going on in your pants?" looks... which I could also still deal with, as I'm an engineer and we're used to people looking at us strangely, muttering and shaking their heads...) this had the more serious effect of very quickly running down the battery. After the third time this happenned I decided the light, as it is, is not suitable for my EDC need.
I then switched to a Streamlight MicroStream, which I bought at the same time as the SWMP, with the original intention to keep in the car. 1xAAA, something like 20 Lumens iirc. This is the singularly most useful light I own. Apart from brightness, it's got all the advantages of the SWMP and none of the disadvantages. It's got a very clever S-shaped clip that allows you to slide it onto your peak cap, from where it functions as a head lamp. I've used it in this manner a lot, when crawling around my roof, installing cctv cameras and wires.
Lastly, I have an O-light i6 Paladin on my bedside table. 2x CR123, 400/80 lumens (selectable by rotating the front bezel). This gets used when the neighbours' dogs sound upset, on NW patrols, and as my general bump-in-the-night light.
At some point I'll invest in an Inforce APL for my M&P VTAC....
:- P