I still have to insist on Cannae. It didn't have a huge effect on history, since Rome was crushed and yet still won the war, but the brilliance of the tactics and the sheer scale of the carnage are amazing.
Hannibal is advancing on Rome, and has just delivered a crushing blow to Rome in a brilliant ambush at Lake Trasimene
http://en.wikipedia.org/wiki/Battle_of_Lake_Trasimene. Now Rome is prepared for trickery and puts its massive army on a huge open plain at Cannae. Hannibal has no choice but to fight in the open with an undersized army. Hannibal is fucked, right? Wrong.
So what does one of the most brilliant generals in history do? He places his weakest infantry in the middle and stays with them. He expects them to take a beating but by staying with them he stops them from breaking. The Roman Army attacks the soft middle, but they push too far in, Hannibal organizes a controlled retreat to draw them further in, and eventually Rome becomes completely surrounded. The Roman Army panics and is completely slaughtered.
50,000-70,000 casualties, only about 10,000 survivors, all within a single day. Among the casualties are about half of the military hierarchy and about a third of the Roman Senate (in the good old days members of the ruling class actually fought their wars for themselves instead of forcing the peasants to fight for them).
http://en.wikipedia.org/wiki/Battle_of_Cannae