I haven't read all the posts, but certainly the key element is using a light which can be powered by lithium primary cells, whether CR123 or Energizer Lithium AA. This chemistry tolerates the temperature extremes and long shelf life much better than other options.
Beyond that, I think a light which offers a quick access to strobe can be very important in a light you will keep in your car. Generally speaking, a strobe feature is one that I have no interest in EXCEPT when walking at night in dimly lit areas where there are cars. Otherwise, I never use strobe and don't want it as a feature "in my way", but if I should find myself by the side of a dark road, stopped for a problem, accident, whatever, I will want quick access to STROBE which can help draw immediate attention to myself and the situation from oncoming drivers... much moreso that a steady on, continuous light can.
For these reasons, I keep two 4Sevens Quark lights in the car, each with quick access to strobe, if desired, and each powered by fully charged, unused lithium primaries.
Of course, in addition, I always have at least one flashlight clipped inside my pants pocket. Usually a D10. And another light or two in whatever case or bag I might be using to carry various needed items. I believe redundancy in lighting is never a bad thing!