An 18650 is hard to beat for performance so I voted “None”. I tend to carry as much as use a light so between the two I’d go for the AA, it has much more cell variety and is smaller diameter for easier carry.
I like 10440/AAA for small & convenient and 18650 for big & powerful. No other size cells have much appeal to me as these two will do everything I want from a light with no others being so much better as to unseat them from their places in my life.
EDIT: Just about all of my favorite flashlights are 1 x 18650 or 1 x AAA. 14500 and 16340 cells are too big to fit on my keychain, and compact 18650 flashlights fit in my pocket quite well.
My sizes mostly favor runtimes with some heft or extremely low weight and volume.
1x18650 just can’t be beat in power-to-size while keeping it light. Far and wide my favorite, wins by a nautical mile.
2xAA offers about the same power of 14500 while feeling better in the hand and lasting far longer.
1xAAA you practically forget is there with you, moreso if you don’t carry it on your keychain but instead clipped to your clothes like I do.
16340 just feels awkward on my hands and is impossible to reverse-grip - I already have to hold my S1 with just two fingers -, and it’s jarringly heavy given the size. The battery life leaves a good bit to be desired as well.
My edc has been a small 16340 light for three years now (currently a modded Supfire S1), I keep trying a 14500 light every now and then because it comes higher current, but it is too long free floating in my pocket (it must be a xlicky), even the ET D25A is too long. Will try a Thorfire TF06 next, but doubt it will last for long.
As much as I like my Olight S1, I still usually prefer AA/14500 cells. It’s mostly due to the thinner aspect ratio. That is, assuming the lights don’t have a lot of extra bulk. I like them in the size of the CNQG brass AA, or the L3 L10. Perhaps even as big as the ZL SC52. However, the ET D25A is pushing it as far as size goes, the TN T10T is also on the large side, and the Kronos X5 is actually bigger than some of my 18650 lights.
About 16340/18350, they usually feel uncomfortably wide. I like the Olight S1, but it’s an exception because it’s unusually small for its cell type. The Rey Ti 18350 is nice too, but that’s mostly due to bling (it’s large for its cell type). However, the Olight S10 is overall bigger than the 14500 lights I like, and even it’s consider small-ish. Going up to things like the UF-602C, those are bigger than a L3 L10 in every dimension, and so is the JB RRT01. However, the UF-602C is uniquely appealing since it takes standard-sized parts easily so it’s great for custom builds. A Convoy S2+ shorty or BLF A6 shorty is even bigger…
So… I guess my answer is either/any, as long as it’s small for its battery type. For example:
DQG Fairy
DQG Tiny AAA IV
L3 L10
CNQG Brass AA
Olight S1
CNQG Brass 18650
Meteor M43
But there are some I still like despite being big for the cell type:
UF-602C
Rey BLF Ti
Kronos X5, X6
JB RRT01
S2+ triples
I guess, for me to like it, it needs to be small or somehow special.
Other than 18650’s, my favorite EDC cell would have to be those ‘NICE’ AA lithium primaries from FastTech.
They’re about 99% as good as Energizer Ultimate Lithium primaries for about a third of the price. A couple of 4-packs of those in my shaving kit and a very small 1xAA light in my pocket, and I’m good to go for a really long time. The added advantage is that they can be used in almost all small portable AM/FM/SW radios and even a lot of cameras - and they never leak!
In general, I am open to many cells but since I live in the Phoenix area I have started to use only primary cells for the lights in my car; either cr123a or aa primaries. The heat will kill any other cell as the temp in my car after work could easily be 130 degrees or higher.
18650s FTW even my EDC is a 18650 based torch( currently a modded S2)
I use some 14500s/RCR123s all arround the house though….actualy the wifey and the kids use those while seaqrching for a lost toy or a dropped ring or something
Oh, one other little detail… When storing a bunch of lights upright, little AAA-sized or AA-sized lights have a tendency to fall over like dominoes when bumped. Especially the BLF-348. This doesn’t happen so much with fatter lights like 16340 or 18350 or throwers or soup can lights. So there’s that.
But aside from the domino effect, I still prefer the narrow ones most of the time.