The ones who actually have to do the dirty work when the government goes to war. They are renowned for being the toughest, and for acting on order without question.
You know those guys the Army always waves around; the Rangers? Well we had a couple of those Army Rangers go through Marines' BASIC Infantry School and couldn't finish the humps (pack marches).
The Marines from 3rd LAR kicked Rangers ass at San D'sMOUT Town; and they even brought more assholes and better equipment than we did.