Marine Corps generally has the reputation for being the toughest. Their basic training program is much harsher than in the other branches, and they're the first-line troops in any military showdown. But the Navy probably has the highest standards for officers--especially if anything bad happens on their watch. And the Air Force lays down very strict standars for who can and can't fly, especially when they have lots of people vying for jobs. I met an Air Force captain some years ago while on vacation in Mexico. He told me was a navigator, not a pilot, because his vision was 20-20 in one eye, but 20-40 in the other. Also told me that, if he didn't make major in a year, he'd be mustered out of the Air Force with no benefits, etc. Pretty harsh.