I read somewhere that 21700 has the best size:power ratio.
I myself still love 16340 
For me, multi cell or 26650 is simply too large.
For a compact like Zebralight SC700, 21700 is ok but on many modells anything above 18650 is simply too big and it won’t even leave the house.
If my brand looses market share, I would improve quality, not printing higher numbers on the wrapper.
I only use a 26650 in one light -Nightwatch NI40 SBT90.2 and find the gold Golisi 26650 provides the most current. I do find the barrel size of this light to be comfortable but I do wish the 26650 would catch up with the 21700 in terms of size /capacity and power ratio. Sadly I expect it will not.
I have read a lot of useful info in this post, thank you all. I happen to be a big fan of 26650’s and have 4 lights that i enjoy with that battery size. I was VERY against getting into 21700 for months because: 1. I vape so my battery setup was already numerous 18650’s, and 2. both 18650 and 26650 would fit my old charging setup. Once I bought the Xtar vc4s and some P42’s / 40T’s for my FW21 pro, my opinion completely changed on the 21700. I now have about 5 21700 lights and really enjoy the size and capacity of the 21700. I would like to see the 26650 stick around (becuase i like the size/flashlight setup) but without companies like samsung, sony and LG working on the R&D, it does not appear they will? I could be wrong? Also, a side note - when measuring the volume of a cylinder it looks like the 21700 has 46% more than the 18650 and the 26650 has 42% more than the 21700. With the 26650 having 109% more capacity than the 18650. That makes it seem like 26650 would make more sense to develop then 21700 to me, but… Like others have said this cylinder capacity math does not take into account how heat dissipation/buildup is effected by cylinder size, etc. I am certain engineers have their reasons to move forward with 21700 over 26650. I just don’t know what i’m going to do with all of my 18650’s! 
I’ve got a number of 26650 equipped lights and they are mostly left at home for use around the house. With the weather getting colder, I may bring one and carry it in a jacket pocket. I like how it fits in my hand, but not my pocket. I like how long they can run too.
I use my 18650 lights quite often. With high quality cells, they perform well and fit easily in my pocket.
I’d consider 21700 cell lights if they can keep size to a minimum and deliver better capacity for their size. It seems that there is more research and development going on in this size so that may mean better performing cells and eventually lights in the future.