The French Foreign Legion has pretty much the most brutal training regimen in the world; it's said that your marches are only finished when your socks are drenched in your own blood.
Really, it's that strenous?
I honestly don't know, though I've been to Parris Island where half the US Marines are trained and it's pretty rough. During the 12 week basic training usually a couple suicides per unit, and at least 18 or so attempted suicides. The whole premise of the 12 weeks is to make the recruits know they're going to die a Marine, and be proud of that.
(they're also a bunch a fucking morons, with a few exceptions)
But as for smaller units the only elite force I know anything about is the Seals, and that's an extremely rigorous program. The vast majority of guys going through their school (BUDS) leave because their bodies can't take the stress and end up breaking down. It's a long, hellish experience, but possibly the most elite unit there is.
Well I'm lookin real hard and I'm trying to find a job but it just keeps gettin tougher every day