I don't know a ton about his college career, but I do know Arthur Ashe was a three time All-American who won a team and singles championship in 1965 at UCLA. Only a few months after that, he was the hero of the US' Davis Cup win, beating Rafael Osuna (a candidate for this honor in his own right) in the first match of the final. Ashe's Davis Cup teammate from that year was another candidate for this honor, Dennis Ralston. He won three team titles at USC, won doubles twice, and won the Individual Title twice.
I think the 1960s was the last time that truly world class players stayed in college for four years, so I think that the guys who excelled in those years were probably the best.
The only other name I can throw out is Pancho Segura. I know very little about him, although I own some autographed memorabilia of his. What I do know is that he was the last guy to three-peat in singles (he did it during WW2), and that he was considered arguably the best player on Earth in the Early 1950s.
Of the guys I have seen play college, though, Johnson is the clear #1. With his winning streak and the four straight team titles, I think he has to be considered #1. Had he won doubles this year, i'm not sure one could have made a case against him.