No question in my mind: Phil Sheridan
Towards the end, even Grant was relying on Sheridan at Appommatox. Sheridan had no quit in him and he was always the aggressor. Even in the beginning at Stones River he gave better than he got. Whether in the Valley, at Five Forks and Sailors Creek, he was, in my opinion a general I would not like to face. He got rid of those generals under him who could not keep up, i.e. Warren. He only had one gear: forward.
ESPN had a program where they matched great NFL teams together in simulation, i.e. Steelers of 75 vs. Bears of 85. I would have loved to see Sheridan vs. Forrest.
As much as I want to dislike Sheridan, He was a heck of an officer.