What if … this Union general had not died?

Military narratives of the American Civil War tend feature the deaths of certain Confederate generals as major turning points.  Albert Sidney Johnston and Thomas J. “Stonewall” Jackson claim pride of place here.  Earl van Dorn … well, he was killed as a result of a different sort of action, so we can set him aside.  But several Union generals also perished during the conflict, and their passing offers the opportunity to discuss “what if?’ as well.  Today’s fatal five include:

Nathaniel Lyon, who was killed at Wilson’s Creek on August 10, 1861;

Charles F. Smith, who died as a result on an injury on April 25, 1862;

Phil Kearny, who was killed at Chantilly on September 1, 1862;

John F. Reynolds, who was killed at Gettysburg on July 1, 1863;

and James B. McPherson, who was killed outside Atlanta on July 22, 1864.

Which death was most significant, and why?  Which of these men, if they had remained alive and healthy, would have made the most impact on the course of the war?  One can offer different answers to each of these questions.

Among those considered were John Buford and John Sedgwick, but in the end I think these five offer the most opportunity for discussion.