The Battle of Tsushima in 1905. Histories only decisive sea battle fought by modern (steel) battleship fleets.
Battle of Gettysburg
Somme.
Gettysburg
Antietam
Charleston
There wasn't a war in the war of 1812 but if you mean battle it is just a matter of opinion
Antietam
diseases
Gettysburg
Chancellorsville
Gettysburg. And the bloodiest single-day battle was Antietam.
Gettysburg is the greatest battle because it was the turning point in the war. Never again would the South have and strategic (big picture) victories or make an offensive move.