The end of the world, I don't need any light, Its the end, nothing matters, fin.
And if it is the case of long term catastrophes, I'll make a DIY tritium powered light source, no power needed and 12.5 till half life 
It would be a light that uses AA NiMh batteries. I have lots of these batteries and a few ways to recharge them. Solar mostly.
I would probably take my TK41 because it has lots of usable modes and I will never want for more throw or brightness. I can use it on 4 batteries if I need to.
However, reading what others had to say about a headlamp, I might take one of my 2xAA lights. I have 4 with another on the way. I have a Niteze headband that will work with any of them. Of the four I would probably take my newest one that just came in today. Shiningbeam Caveman. I would probably pick the Caveman over the Klarus P2A or Jetbeam BA20 because it has an extra mode of 8 lumens for much longer battery life. Also being an XM-L it has more spill.
Tough choice. I'm going to go with the TK41.
Looking at it logically,a 1AA light that can take any Kind of cell would be the most practical. AAs are the most common battery on the planet and you can harvest 14500s from old laptops and even some electric cars (Tesla).
Out of the lights I own, I would pick the Klarus P1A even though I my P2A is brighter. There may be a time when I can only find one good cell.
Thats a good point.
This is not actually mine but hopefully will be soon!
Second this. Power / runtime & versatility, reliability + size.
Sc51
Pretty sure that senario is covered
I am the light of the world. Whoever follows me will never walk in darkness, but will have the light of life.”
The Solarforce L2i .... 3 mode low voltage drop in ... 0.8v - 4.2v
Runs , AAA - AA - 10440 - CR123A - 16340 - 14500 - 18350 - 17500 - 18500 - 18650
Know any other flashlight that can run so many different sized and Voltage range batteries ?
End of the world? . . .
It's gotta be able to mount on a weapon 'cause . . . well, some kind of weapon will be my first priority. Also, wouldn't it have to be AA? I'm just thinking of the most likely-to-be-found-while-(preventing)looting.
Probably gonna be a lot of shooting and looting.
IcanwaitFoy