Never sure about the superlative "best", but this Primary Weapons Systems is a pretty awesome armadillo getter. This 9.75" upper has a long-stroke piston ala AK47. It is quite versatile with either red dots or scope.
Build list:
PWS Mark 109, 9.75” 1:8 twist
Surefire WarComp QD muzzle device
Surefire 762 SOCOM2 suppressor
Aimpoint T-1 , LaRue LT660HK QD
Surefire X300 Ultra
Trijicon RMR 3.25 MOA
LaRue LT872 QD RMR mount
Schimdt & Bender 1.1-4X Short Dot on LaRue QD mount
Precision Reflex Gas Buster charging handle
Lower: Limited edition GPI Legacy Arkansas Ranger M4A1
Magpul MAID grip
Leitner Wise Buffer retainer
Magpul aluminum trigger guard
Geissele SSA trigger
Battle Arms short throw ambidextrous safety
Wilson Combat match grade small parts
Wilson Combat buffer tube
Trident Tactical buffer spring
Law Tactical Gen3-M folding stock adapter
Rogers Super-Stok & cheek pad
Rogers single point harness adapter / sling QD
Phase 5 bolt assist lever
Rogers Mag-Grip
Tactical Innovations Teflon tension screw
Fabricated stock-ELS-spare magazine carrier system
Rogers Magazine Grip

