Oh, and you cant really claim which SOF unit is best. They all have their niche's and theyre all the best in the world at what they do.
If you have to pick one, pick one based on what suits you.
You like Maritime environments?
-Try to join the SEAL's or MARSOC.
Dont like water?
-Army Rangers or Green Berets, or SFOD-Delta if you make it into the Green Berets and are badass enough to pass selection. (At least I think that the Rangers are now considered part of SOF, just not considered Operators like the others. Pretty sure the Rangers bridge the gap between the small Operator teams and the full blown infantry units.)
Like to jump outta planes to rescue lost pilots?
-Air Force Pararescue.
As far as other countries go, I couldnt tell you. But they all have similar purposes and training, with the simple exception of money and equipment, which is one reason why our units are as highly regarded as they are.