Princeton Tec 20. Here's why:
1)It can use Lithium, NiCad, NiMH, or Carbon Zinc batteries.
2)It adapts to a huge variety of cheap readily avilablle PR based bulbs.
3)Waterproof, not just water resistant.
4)Lightweight.
5)Inexpensive. $9.00 or so on sale, batteries and lanyard included.
6)It can be converted to LED with the Matrix Module.
7)It can be quickly swithced to a Luxeon light (several colors) with 2 different types of PR based Luxeon bulbs available.
8)It can be adapted for use as a bike helmet light with a little velcro.
9)Smooth and textured reflectors are available.
And here's some subjective observations about this light: It feels well balanced and carries well in the hand, PT customer service is outstanding, and the available case colors are pleasing to the eye.
All of the above also applies to the PT 40, except it is a little more expensive and cannot use Matrix Module (unless you want to run dummy batteries) which is designed for 2 cells.
Granted the PT 20 may be a tad too big for EDC but when you only have one light everything is a compromise.
I challenge anyone to find a more versatile light.