I can't agree that shots that started battles were in and of themselves significant since the battle would, in all cases cited thus far, have occured anyway.
The arguement for the immortal Jackson is interesting. Had he been present at Gettysburg he might have been successful where (was it Ewell?) failed in taking the hills south of Gettysburg, thereby turning the Union position. While it might have led to ultimate Northern defeat in the war, such does not automatically follow. Much of the Union army was still approaching, and Meade could have withdrawn to battle again on better ground. Grant still would have won at Vicksburg. There was still time before the 1864 elections for other victories to keep Lincoln in office.
In the Civil War context that shot that pinked Gen'l. Albert Sydney Johnson behind the knee, causing him to bleed to death at Shiloh is at least as significant as the one that got Jackson. Second in command Beauregard did not press the attack. Union reinforcements were gotten up river and the following day's counterattack gave Grant a victory. Had Johnson lived, Grant most likely loses, perhaps has his army destroyed -although that is less likely-, which severely handicaps his opportunities for later advancement given the already extant rumors of his drinking. No Grant on the Mississippi does Vicksburg get taken? No Grant in Virginia in 1864, does Lee get held in place and worn down?
Of the four presidential assassinations, Lincoln's had the greatest impact on US history. Regardless of how successful he could have been in promoting his let-'em-up-easy philosophy, the fight over it would have created a different chain of consequences. In another 50 years the tragic end of the sickly and vacillating Kennedy will be as dim as Garfield's or McK's.
Unlikely that the Kingfish would have been elected, although, had he lived, he might have drawn enough votes from Roosevelt so the Republicans won.
The non-shot at Washington, if not apocryphal (sp?), is certainly significant. But is, after all, an event that didnt happen. (What if Osama bin Laden had been eaten by that White shark while he was swimming in the Indian Ocean on a family vacation in 1974?)
Sadat was most likely dead sooner or later, he had stepped too far out of the box for most in his world to tolerate.
I think the shot/shots that did for the archduke Ferdinand get my vote. Unlikely that he would have been assassinated had Princip failed, based on subsequent survival rates of other world leaders following failed attempts. War might have come over some other issue, since the Kaiser was so hot to prove he could whip his cousins and uncles, but such an outcome is much less certain sans the dead archduke.