I am starting to EDC a ZebraLight 502 and I like it a lot. Right now it perches on my helmet during the pre-dawn bike commute. It will be carried regularly one way or another when i am not bike commuting.

It is a single AA light with three levels of brightness, which are adjustable. It is tiny, powerful, well made and dependable (so far), but not cheap (natch!)

If I could pack only one light source, it would be a headlight - far more versatile and useful than a typical handheld flashlight