The answer appears to be W T Sherman so I'll go with that.
However, aside from cavalry officers (Kilpatrick, Custer etc) who needed to wear boots because they were riding hard, it's difficult to find Union general officers who didn't often wear shoes rather than full boots. They were on foot more than on horse and walking in boots was difficult for all of 'em.
Sherman wearing boots:
Grant not wearing boots:
Here's a fun excerpt from the 1887 Journal of the Military Service Institution of the United States:
"Uncle Billy, as the men called him, didn't like the feel of boots when he was riding his horse Sam, named after Grant. He said it was to hard to think with boots on." http://www.funtrivia.com/submitquiz.cfm?quiz=299854
William Tecumseh Sherman. Uncle Billy, as the men called him, didn't like the feel of boots when he was riding his horse Sam, named after Grant. He said it was to hard to think with boots on.