That would be a tough one. I would have to give the safe answer of John Bachelder, who's general regarded as the foremost historian of the battle in the late 19th Century. Lot of early interpretations of the events can be tied back to him. Frederick Pfeffer also deserves a shout-out, who eventually came to own Ziegler's Grove after the battle and built one of the largest private collections of Gettysburg relics. His descendant, Erik Dorr, operates the Gettysburg Museum of History which is built around said collection.