For a true pocket light with commonly available batteries, I would stick with a single AAA or AA battery. I run lithium batteries in all my crucial AA or AAA flashlights for long runtime, long shelf-life, and no worries of leaking and corrosion.

recommendations:

Arc AAA-P, bright, very small, rugged, decent runtime.

Fenix L0P (AAA) small, very bright, short runtime.

Gerber Infinity Ultra (AA), rugged modest output, long runtime.

Fenix L1P (AA) Very bright, short runtime.

Fenix L1T (AA). I don't have any personal experience with this light, but the option of 2 levels may make it a superb choice.

Inova X1 New style (AA), good brightness, good runtime. good all-around light. I put a light film of conductive grease (penetrox or Ox-Gard) on the battery contacts and the end of the body (with the end cap removed). I don't have any problems with the momentary switch.

I use conductive grease on all electrical contacts I can access on all my flashlights (battery contacts, bulb contacts, tube ends, etc). I have had increases in output in some cases and never hassle with intermittant output (having to shake or pound the light to get the output to full).

Not my first choice for a pocket, but some don't mind carrying a 2AA light.

MiniMag with Nite-Ize conversion. Inexpensive, decent output, flood light, long runtime.

The L2T may be another good solution (no first hand experience) with the 2 levels of output. The head is smaller than a minimag making it more pocketable.

For more info than you know what to do with:
http://www.candlepowerforums.com
http://www.flashlightreviews.com/index1.html

Good luck, TR