MK20 - 5.56 , fast fire rate lower caliber
MK14 - 7.62 , more recoil but packs a punch.
MK18 - 7.62 , unlike its counterpart (MK14) the recoil and accuracy are much better, with a good fire rate.
MK1 - 7.62 , great accuracy great recoil control however it has a slower fire rate that's the MK14 and MK18
My personal preference would be the 14 or 18 due to the much lower price but the devastating punch that it packs!