As an old frogman, SAS was by far faced the most hazardous missions given the (Northern Ireland) issue at the time. Very !??@ group of people.
The SEALs were and are by far the best small unit operators around. Lack of human flesh to go around produces high turn over rate. No Seal wants or does rejoin the regular navy after being in Spec ops. You are eihter in or out. (down time for training was short)
Army Rangers well they do not spend 20-25 years in the Rangers. Its is a short tour but a hell off a grind to make it but its back to regular army afterwards. They are quite a fit group.
Marine force recon, they are good but they are not Seals.
Green Berets? are not the same as depicted in the Old John Wayne movie! They are army specialist. They are not butt kickers like the Rangers.
I was impressed with the Aussie SFAS units.
But the most skilled and useful trained is the Air Force Pararescue!!