I have a SkyRC MC3000 and love it dearly, but if you’re not willing to spend that kind of money, I have heard very good things about the Vapcell S4+. I don’t have any info (neither good nor bad) on the Nitecore D4.
wait which light uses 21700? i missed what you’re saying
i have 18650 flashlights from sofirn mostly
Sorry for the confusion. I use a 18650-eating Wurkkos FC13 as powerbank, it works great but for a small problem that I have now solved. Unfortunately I don’t own any 21700 lights (at least yet).
I know that most (all?) of the Hanklights don’t have powerbank (I think most/all don’t even have a USB port for recharging).
As for the Wurkkoses, I can vouch for the 18650-eating FC13 from my direct experience, but I believe at least the TS25, the TS21 and the TS30S Pro (all of them eating 21700s) would also work great (but as I don’t have any of them, I urge you to ask people that have one and check that my belief is true in fact).