For the cheapest and brightest 18650 headlamp, I would still go with the Boruit D10 Headlamp. BLF review here: Boruit Headlamp Review D10 (aka. EHL0628) - King of the Budget Headlamps?
For less than $15 (BORUiT D10 XM-L2 LED Headlamp) and includes USB charging and a decent UI, it’s a bargain!