Surpringly my FW3A. I’ve opened it numerous times for modding (it’s got XP-L HI 5Ds) and similarly to djozz I wouldn’t rely on it unless this was the case. It’s never given me a problem… I guess I got lucky
Also, my 5600k sofirn C01 has been on my keys for a year now. That’s a reliable light in terms of both durability and battery life, especially with an energizer L92.
My “takes a licking and keeps on ticking” lights with beat to crap appearance are;
Olight s1
Zebralight sc62w
Four 7’s quark (aa)
ITP a3-eos (aaa)
Fenix Eo1 (aaa)
Fenix pd35vn TAC
Fenix HL60r headlamp
All have many hours of hard labor and lots of recharging.
None of my other lights have been through this much h€ll.
So far none of my Fenix, Zebralights, nitecores, 4sevens,and maglites have failed me. Ive had issues with an AAA clicky from coast. Threw it away.
6P bored for 18650 with Pflex Pro drop in and McClicky switch. Also has the beefy Lumens Factory SS bezel ring and Solarforce pocket clip. 
These days, the only lights that are unreliable are the cheapest ones that have poorly soldered connections in them, or weak body protection to prevent moisture ingress. The most reliable flashlight is going to be a single mode twisty. Can’t get more reliable than that!
I have found my FW3A to be “temporarily” unreliable, due to battery tube connection. But that does get cleared up with cleaning & re-tightening. And that generally only happens after a battery change, if it does occur.
My oldest flashlights are Fenix & Olight, from 8 to 10 years ago. Plus an old NovaTac 120P. They’re all still in service.
Of the 40 or so I own, the only one I’ve had issues with is the Lumintop FW3a Ti, so for me everything except it has been 100% reliable.
Both single mode twisties but not budget.
I'm not really qualified to answer the op. I don't push my flashlights hard enough to know which ones will fail. I lost my Kronos X5 the other day after dropping it on an asphalt roadway, so I can't pick that one.
On paper, I would choose one my ZebraLights. I have an SC62w and an H600Fd III. Potted electronics are the reason, but I have no test data to back up the choice.
Ultimately, test data is the only real way to answer the op, but barring that, and my ZL selections, as well, I liked the answer given by Random Dan.
I got no evidence to back this up, but it sounds good to me.
Fenix hl55
5 years used daily, used for rock climbing, bashed agains the rock all the time, been through the washing machine a few times
Still going strong
If I did have to pick just one flashlight that I’d take with me on a survival mission, it would be the SC64c LE. I prefer the Anduril UI, but the ZL UI is so well proven. Also, there’s tons of positive testimony on reliability. Plus, ZL excels at efficiency. The potted electronics is also a major advantage.
FW3A
i know it got a bad rap
i have had zero problems though
usually i break switches, or inductors fall off, or just mystery fails
i’ve dropped it many times, it is scarred but still working
i consider the switch, particularly, A-1 reliable
wle
For me the most reliable is the Convoy S2+ and C8
The more complicated the less reliable it can be. I saw a lot of L16 driver errors and bad Nitecores Armyteks and Imalents too. And the most important if anything goes wrong you can buy anything for an S2+ or a C8 and repair it for 4-5 bucks max.
I have too many…
SF C3 with Malkoff m31w
SF G2 with customlites p60/219
SF 6P with customlites p60
SF G2 with Malkoff m61NL
Streamlight polytac
Streamlight Survivor
Surefire G2L-FY
Surefire G2X
Lumens factory seraph/Malkoff m361w nichia
Lumens factory seraph 9v with 4 cell extension and turbo head
Armytek wizard pro
Fenix E01
Sofirn c01
LRI photon freedom
Peak eiger
But that’s just listing my “most reliable” as requested by OP… probably need to sell some but it’s hard to pick!
Better question is what is unreliable. Most of mine are reliable.
ZebraLight SC63w. I have a couple Twistys that I use at my bench that have switch bounce/ mode skip that are really annoying. Still looking for the perfect bench light. 
A light has to be more than just reliable though.
Yep.
The vast majority of my flashlights work perfectly.
A few of them have stopped working after being dropped, but that is to be expected.
Any of my lights with the word Malkoff in them.
Oh, I will start with lights that failed on me:
Zebralight SC600 - (switch ) - sent bank to China at my expense, and another $30 return shipping.
Nitecore EA8 - sent back twice to China, both times at my expense. First problem was light stays in a very dim mode. Next trip was due to non-operational. Eventually Nitecore replace it with a new EA8. Both times return shipping to me was on them.
Fenix TK41 - sent back to China at my expense due to a failing, intermittent switch…came back at their expense but arrived still not working, eventually replaced with a ‘Test’ unit FD65.
Fenix TK70 - failing intermittent working switch, did not bother to have it repaired.
Fenix headlamp — can’t remember the model, but it was single 16340 or CR123. Did not bother to have it repaired.
Xtar WK something —- single AA light. Did not bother to have it repaired.
Ironically, the lights that have never got into problems are the cheaper, mechanical rear clickies.
Bad things:
I use to repair and diagnose failed and returned flashlights for a hungarian flashlight webshop and I met various defective lights.
Acebeam H15: factory not soldered a resistor correctly on driver.
Acebeam L16: driver lost modes. Only turbo if I remember correct. Acebeam sent new driver
Acebeam T30: one diode failed in XHP35 HI and that messed up driver UI
Armytek Wizard pro magnet usb: Charging not work. Driver problem. It does not connect battery when charger attached (Yes the tailcap untwisted half turn)
Nitecore HC65: very abused flashlight. Battery leaked and flashlight only flashing blue led when charger connected. Otherwise no life signs.
Nitecore P12: Lot of use and water leakage plus multi board driver solder joints broken
Astrolux C8: bad switch 2 times
Imalent HR70: Completely burned out charging circuit
Imalent DN70: contact problems with tube
Imalent DT35: sensitive to tube tighting. Sometimes led not light up but display said 2200 lumens
Niwalker BK-FA 30S: squeeking noise in turbo and most of the time shuts itself off. Cleaned all contacts and resoldered everything but none helped. Driver problem
Few cheapo Boruit completely dead
Utorch S2+ clone: driver spring caused wrapper damage on battery and a short so both springs melted
User errors:
Convoy L6: No not the light fault. The owner removed reflector and managed to damage and rip off dome and reflector was full of fingerprints.
Manker U23 and E02: user electronically locked switch and can’t use unlock combination click—wait 2 sec—double click—wait 2 sec—triple click
Imalent HR70: head loose on tube so mechanically locked out
Nitecore MH20: tried to use flat top battery but MH20 works only with button top
Soooo I found that Convoys have the least problems. He does not sell and I don’t have any Zebralights and a few brands so take this a grain of salt.