In all of history what military unit was the ' toughest ' ?

Question:

In all of history what military unit was the ' toughest ' ?

I realize this is an 'impossible' what if query. But if your back was to the wall what historical 'men of arms' would you like to be shoulder-to-shoulder with? My choice would be the 2nd/4th Marines April 2,1945 Okinawa. [Honorable mention: The Royal Highland Regiment of Canada 'The Blackwatch' Vimy Ridge April 9th, 1917.]

Best Answer - Chosen by Asker: In the past, probably the Sacred Band of Thebes or the Spartans, or Alexander's army, but the Greeks have obviously lost their edge since then. Honorable mention also to the men who defended Roark's Drift.

In the present, if I ever have to jump in a foxhole I hope there's an Israeli soldier in there with me.

Of course, you should also consider that a fighting unit is only as good as its leader.