U.S.M.C Force Recon
Though now that I think about it there really isn't a best answer for this question because all special operations military units are designed and trained for different things. Delta force does hostage rescue and extraction, U.S.M.C scout snipers are the worlds best snipers, Force Recon operates ahead of the main force either gathers intel or sabotages defences, SEALS are quick strike raiders (Rangers are as well but they aren't as good as SEALS), and Green Berets work with/train local military forces and run missions with other military units. I don't know what the SAS do but all the units that are listed are among the best in the world at their job, but none of them are the single greatest operations group.
Though now that I think about it there really isn't a best answer for this question because all special operations military units are designed and trained for different things. Delta force does hostage rescue and extraction, U.S.M.C scout snipers are the worlds best snipers, Force Recon operates ahead of the main force either gathers intel or sabotages defences, SEALS are quick strike raiders (Rangers are as well but they aren't as good as SEALS), and Green Berets work with/train local military forces and run missions with other military units. I don't know what the SAS do but all the units that are listed are among the best in the world at their job, but none of them are the single greatest operations group.