Get out of the mindset that there is a “best” anything with firearms. There is more or less suitable for YOU. The best stock for an 18” build for me is a Magpul MOE Rifle Stock. The best stock for someone else might be their cheap piece of garbage carbine “M4” stock from some no-name vendor for $40.
There are several more variables like gashole size, gas system length and ammo type(s) to be used that drive the considerations like buffer weight, spring weight and possibly rifle vs carbine vs VLTOR A5 receiver extension selection.
There is no “best”.