My personal preference is:
Scar-H if you use FMJ, Red Dot, Holographic, ACOG
Tar 21 if you use Silencer, Grenade Launcher
This is with Stopping Power on all.
Also, the Tar 21 has a unique sight, but I forget if its Red Dot or Holographic/
I don't use Extended Mags, Thermal, or Shotgun, so can't help you there.
Hope this helps.