I carry two small LED flashlights. One of them is usally a 2AAA penlight, like the Streamlight Protac 2AAA, Streamlight Stylus Pro, or Pelican 1920. I carry a 1AAA light as a backup.
The Protac 2AAA has both bright and dim levels so you can avoid blinding yourself when doing up close work.
The Mini Maglite 2AAA is plenty bright for a penlight, but twisting the head to control it usually requires two hands. A pushbutton operated light like the Streamlights is easier to use.
When I have a specific task, I use an Energizer Romeo Molle light. It works great because it has strong magnets recessed in the clip. The clip is free to rotate so the light can be oriented in several positions. I've stuck the Romeo light to the chassis when working underneath my truck. It is powered by a single AA battery.