Rifleman Tim Murphy, firing from a tree, mortally wounds British General Sir Simon Fraser and changes world history.
Thanks to Typeay for this list of The Top Ten Snipers in History.
Unfortunately, the listmaker leaves out the rifleman who fired the most important single rifle shot in history -- Tim Murphy.
At the second battle of Saratoga as American troops were falling back under a vigorous British attack, Murphy, on the orders of General Dan Morgan, shot British General Sir Francis Clerke and General Simon Fraser. Fraser was the single most effective British troop leader on the field at Bemis Heights. When Murphy mortally wounded him from a distance estimated to have been about 300 yards, the British attack collapsed and the fate of "Gentleman Johnny" Burgoyne's army was sealed. Though Murphy served through the rest of the war, his shot downing Fraser was the single most important shot he -- or any other rifleman -- ever fired in battle.
With the British defeat and subsequent surrender at Saratoga, the French decided to come into the war on the Colonists' side. Without Tim Murphy's incredible shot (from a tree, no less) the history of the Revolution likely would have been dramatically different.