ML50LX LED 2-Cell C
Maglites just seem to last. The Mini-Mag isn't as durable, too small and lower lumens. The D cell lights are too big. I think for a common, easily found cell, the 2C cell ML50LX is the Goldilocks flashlight--not too big, not too small. It is a compromise light. Not too big to fit in a pocket or purse, but powerful enough to have good illumination and throw. It is the "CCW" of flashlights, the "pocket pistol". Hmmm, shoulder holster carry??! Uhmm, probably not. Appendix carry? Nope. Open carry, maybe.
Anyway, that is what I came up with, and why.
Now the only reason that I can think of as to why you could only have ONE flashlight for the rest of your life, would be something like an off-earth excursion, like a Mars mission, where you are size and weight restricted, and so limited to what you could take. However, with something as vital as light, redundancy must rule. Three is two, two is one, one is none.