Sorry to say, but you guys are missing the best of the best here.
The SAS, Delta, ST6, etc. all are elite groups, but they still are not the best... Not even Delta, who's probably the best out of those three.
CIA has their own paramilitary Special Operations Group, or SOG. They have gone in before other soldiers in every war the U.S. has been involved in since Vietnam (it might be since WWII, but I can't remember).
They are easily superior to Delta or SAS. In fact, CIA SOG recruits from Delta. So to get into SOG, you might have to get into the Army Green Berets, then get asked to join Delta, make Delta, then get asked to join CIA SOG, and then actually make CIA SOG. You'd have to be beyond bad*ss at that point.
From the book "Special OPS: America's elite forces in 21st century combat":
"Highly classified, the SAD is regarded as the preeminent special operations unit in the world. Members are the elite of the elite; "the best period." This results from the sources from which the organization recruits its members: Special missions units (SMUs); such as Delta Force and NSWDG (United States Naval Special Warfare Development Group)..."