I highly recommend the Quark by 4Sevens.

http://www.4sevens.com/index.php?cPath=297_305
or
http://www.4sevens.com/index.php?cPath=297_355

They are remarkably small, light, powerful, long lasting and nearly indestructible. They have several modes which are practical every day from blinding bright to nightvision-saving low and a few survival oriented modes such as SOS and becon which last for weeks. They can tailstand; they can be mounted as a headlamp or weaponlight and the diffuser turns them into a lantern bright enough to light up a large room. Finally, for $20 you can buy additional bodies to change them from a CR123 to a AAx1 or AAx2.