I'd choose the Magpul manufactured Bushmaster ACR Buttstock for the win. Unfortunately, Magpul has apparently ceased production so the used market may be your only hope.
My second choice would be the new B+T Collapsing and Folding Buttstock as found on the "Pro" models of the APC family. It appears to be a reasonable facsimile of the ACR stock.
My last choice from an ergonomic and aesthetic perspective would be the AR Tubular stock with whatever Cheek-Riser works best and a folding adapter. YMMV....