I've read that John D. Rockefeller was by far the richest American ever if you measure it as a percent of the nation's wealth in his era. In fact, this link by Forbes shows that he was 1.53 percent of the entire U.S. GDP. That's pretty impressive. Bill Gates, by contrast, is 0.58 percent, which is still impressive, but he's a housemaid compared to Rockefeller.
http://www.forbes.com/asap/1998/0824/032.html