These rankings tend to be subjective, IMO.
Special Forces Units tend to recruit the best of the bunch from the military and train them even harder. These soldiers are generally very good in their game. However, resourcefulness of the unit is another factor altogether and American special forces have the edge in this respect.
Operation Neptune Spear is an example of tactical brilliance, resourcefulness and capability of American special forces to conduct operations in the heart of hostile territory even in night-time conditions with minimal losses. As a Pakistani, I feel a bit off to mention this but this is a reality we have to live with.
Regardless, SSG is a formidable bunch in its own right. I am always interested in learning more about this force.