Best of the best?

[Bra]

The US Marines are better trained and better equipped. Having more money than everyone else tends to be quite valuable to whichever country... has more money than everyone else. To be fair, though, you should have cited the Special Air Service or something, since they're far more frequently compared to the US Marines even though their purposes aren't identical. The SAS is of a higher caliber than the Royal Marines.

'Sides. It's all about JTF-2, anyway.