This is just a hypothetical situation.. (well at least until later this year) 
The apocalypse is here and you can only take 1 flashlight, which one is it and why?
I guess no one because i won't be able to see anymore hahhahaha
Seriously, i would go with the fenix tk35. Compact, powerful, good runtimes, good quality.
Probably my ITP A3 eos due to its small size, simple design, and it runs on one AAA.
CR123As are going to be hard to scrounge in an emergency. You could carry a supply with you, but thats added weight and if its the end of the world, eventually whatever supply you could carry would run out and you would need to scrounge for more batteries. If its the end of the world you would not be able to charge your 18650s unless you have a solar charger but there is a good chance there may not be adequate sunlight for quite some time.
C3 w/ extension
enough throw to spot deer (w/ two alkys), not too bright for close up work.
I'd like to build a direct drive QTC light for the zombie apocalypse - no circuit to fail but adjustable. Just don't have any QTC yet!
If a solar charger (or whatever other power source can be obtained) a V60C, for a lowish low, and enough high, and anything in-between, with enough bulk for both output and or runtime.
Otherwise a TK41 with good LSDs, because there will probably be more usable cycle life from NiMH than Lithium.
You are right, you can find a lot of AA, but i guess you mean alkaline(no lithium or nimh, the nimh have the same problem of the charger,lithium can't be stocked too like cr123) .But there comes the problem of the energy density, with a AA light the alkaline would not last long like one with cr123, in terms of the same flashlight output and efficiency. And you wouldn't be able to find alkaline all the time too in that case! Ok, anyway, if i changed my mind about the fenix i would take the zebralight H51! Freeing your hands in difficult situations is very important!
So prepair yourselves and keep always some charged batteries and primaries!
BTW, i have a good stock of cr123, as well as lithium AA and AAA!!!I am waiting for the armaggeddon, but probably i am mostly crazy!
No it isn't TEOTWAWKI and I can have all the flashlights I want, thank you very much! 
OK, I 'll try to get over it 
After the apocalypse, things will probably be tough, wet, dusty and maybe extremely hot. Also, my guess is that electricity will also be out, everyone would try to help their families, thieves will have none to stop them and a guy with a bright flashlight is like a "welcome, I have stuff you need" message. So I will need a real low to go unnoticed and a real high to check my whereabouts if needed. It would be nice for this flashlight to be able to be used as a blunt weapon, so either it should be big or have crenelated head / tail. It will also be good to be able to use 18650 batteries (I have ~25) and cr123a (I have ~30) and rcr123a (I have ~30) because, you know, no electricity, when batteries are gone, are gone for good.
So it looks like I don't have the perfect single flashlight for apocalypse! And now I know I must find one 
OK, I might just take my L2T with MF/UF XML dropin which I use every single day and I learned to trust or my TK35 which covers most of my requirements above.
actually, you're right, a headlamp is wisest, but I do have a band to stick my C3 in, and a clip on it, and one wrapped with electric tape for mouth duty without chipping my teeth 
I'm not worried about 'finding' AAs - I have spent hours...actually days...harvesting packs and have over 150 good (>800 mah) nicads, >40 nimh (from 1600 to >2k mah) and about 100 alkalines (utilitechs). I also have two solar battery chargers.
I'm ready baby...bring the zombies.
I wonder if they would outlast one LED's claimed 50k hrs 
Leds if properly used(low power settings, for example for xp-g, xm-l less than 20 lumen) would last for ever.
no to get too far off topic, but I have a "freeplay" radio, it can run off crank power, has sw/fm/am, it will run off the solar battery charger and it has a light on a cord that you can plug into it, that i just realized I need to mod to that....xml
A 501A because I can mount it to any of my firearms it also fits in my Ultrafire headband. Battery wise I can use 18350s, 16340s, or CR123s change the body tube to a 501B and use 18650s instead of those or with a low voltage drop in even AA or AAAs. Plus with my custom 5 mode drop in it will run for weeks on moon low with one 18650 if I need to.
Jetbeam BA10
With a pocket full of AA batteries, this thing will probably run longer than I can.
A 6D mag.
At least it has a defensife use. Due to the total chaos aorund i might even get a kick out of it. (or some "clubbing" session)
I just had one of those idea moments... 
Does anyone know where to buy flask/flashlight combination ?
Kinda impossible to pick just based on a general "end of the world" statement. I guess my Olight m20s since my Fenix tk10 is having switch issues. I have gasoline powered generators, hand crank, and solar.
I guess it depends on what happened though, is it zombies, nukes, government breakdown, etc.......
If I have to go on the move, I might pick my Energizer Hard Case headlamp, since it uses scavengeable batteries.
If it is truly everything is over, then I guess that I'd grab the Olight with a bag of fresh primaries. I'd save it for emergencies though and just use open flame for most stuff. I can make a decent torch with basic yard debris and virtually any cooking oil, motor oil, diesel, alcohol, can be used in a basic wick-type lamp, you can also make candles....etc, blahblahblah..
Here comes the fanboy again, but I don't even have to give it a second thought.
H51w
I'd prefer the SC51w, but if its all I got, I'd rather be able to use it as a headlamp and the H51 is more easily adjustable in the headband.
I would get my 2AA Ultrafire C3 light. It can run off either one or two AAs. Versatile and small.
I can't believe no one has said the Fenix E01 yet. You could steal batteries from almost anything.
itp sa1 would do me well..bright, ipx8, pretty nice low, strobe, small and can clip to the bill of a hat if need be..when my 14500's run out or I am unable to charge them via my car charger then aa's would be the easiest to salvage..