As far as the US military, its actually hard to quantify, but of all the units under SOCOM, the US Army SFOD- Delta, and US Navy Specwar Development Group would be considered the most elite.... Just like there are 3 regiments of Rangers in the army, there are (I think) 8 different SEAL teams... Even numbers based on the East Coast, odd numbers on the West Coast.... I am not that familiar with the Navy and the Special Warfare units, so please excuse me if I get the details wrong.... I believe the Naval C/T team is now known as DevGru (Development Group) rather than Seal Team Six.... Both units are near the top in world for ability and I don't think you can say one is better than the other.... Both units work closely with one another, as well as with other C/T teams throughout the world...
Its more about the man than the unit.... All the units are supremely capable, and extremely selective on the personel they allow to join the units... The selection course for each is both physically and mentally demanding.... but passing the selection course is only one step to joining the units.... Its often said its harder to keep beeing a member for any length of time then passing the selection course... its extremely demanding....
The Air Force Combat Controller is also an extremely elite team, frequently made up of former members of other units under SOCOM.... I just don't think one can say which is considered the most elite.... All the members of such units have undergone a tremendous amount of selection, training, and personel sacrifice to get where they are, and my hat goes off to all of them...
Dave
Edit: As a side note, someone mentioned the German C/T team GSG-9.... While one of the best C/T units in the world and a bunch of bada$$ shooters, they are not part of the German military, but the border police....