Everyone's got an opinion. Mine is PWS. They make a long Stroke Piston AR (think AK). Super reliable MilSpec and it comes fully dressed, with Magpul furniture and sights, BCM gunfighter charging handle, their FSC556 muzzle break and enhanced buffer tube and Quad rails. Plus there is no gas regulator to mess with like most other piston ARs.
I snagged mine for $1,500 plus plus.
For those thinking that gas piston is a passing fad think AK, M1, HK and Sig. They're all using some sort of a gas piston for QUITE some time and it's worked for them.
SIG 1911 .45 ACP
SIG P229R 9mm
Beretta BU9 Nano 9mm
Beretta 84FS .380 ACP
Ruger MKIII 22/45 Target .22LR
PWS (Primary Weapons Systems) MK114 5.56