existing or fictional?
existing i suppose a M6, but with factory rechargeable option.
fictional it would be a light with a built-in nuclear powerplant for 3000years of runtime on max brightness.
Max brightness would be about 10000000000 (10 billion) lumens
analog type brightness setting (stepless).
two-stage switch (push for momentary on, press for constant on)
wall of light to laser kind of focusable head
setting for color temperature, anything from 1k to 16k
about the size of a current 2xCR123 light.
infinitely reliable (even one failure per billion lights wouldn't be acceptable, i want 100%)
made from unobtanium (fictional material that becomes stronger when exposed to pressure, heat, or any other type of force/power)