Toss up between gettysburg and the Normandy landings.
Gettysburg is so great because of the piecemeal way it formed. You had a bunch of troops milling about, and heres this central junction, and it just... happened. A few shots here, a few there, and the armies just kind of formed up as they arrived. Which led directly to Chamberlains triumph, because if the union left had been properly secured, he never would have been in danger. His quick thinking and the discipline of his troops truly won that battle.
Sherman's march is also up there, just cause he was such a cold S.O.B.