For my small (able to fit in a BDU pocket) kit, there's either a Gerber Tempo or Rayovac penlight. They each use one AAA battery and are single-mode. The Gerber is a tailcap twisty at 8 floody lumens. The Rayovac is about the length of a pen, reverse clicky, and 3 tight lumens. The Gerbers cost about $7 and the Rayovacs are $3 at WalMart. Both have proven to be reliable and take up very little space.
My larger kits carry one of the Rayovacs listed above, plus a Brinkmann 3-mode headlight (now re-branded as Defiant) along with one of these: Streamlight MicroStream, Inova XS, or Fenix E05.
My home kits house one of the Rayovacs, one of the headlights listed, and a Defiant pen light (two AAAs, 23 lumens, reverse clicky).
All of these lights take AAA batteries, so I can just throw some fresh cells in the kit when needed.