In no real order: Pernell Whitaker Willie Pep Nicolino Locce Ricardo Lopez Harold Johnson Wilfred Benitez Miguel Canto Muhammad Ali Roy Jones Jr. Floyd Mayweather jr.
Ricardo Lopez wasn't the kind of guy who made guys miss shot after shot after shot like Sweet Pea or Benitez, but because he was just so darn good at integrating his defense with his offense, he rarely got caught flush. I think that ring generalship, a strong one punch knockout power, a high guard, good foot movement, and great head movement count as great defense. Therefore, I put Ricardo Lopez at number three behind Benitez and Sweet Pea. I haven't seen enough Pep to say for sure.
One fighter who has been overlooked in this thread in regards to his superb defense is "Gipsy" Joe Harris. For a man who was for the better part of his boxing career completely blind in his right eye he sure was a hard guy to land a clean punch on. His one-sided victories over tough Stanley Hayward (TKO7) and the very competent Curtis Cokes (W10) were defensive masterclasses and he made both men miss so often that they were made to look foolish... pretty impressive for a one-eyed fighter if you ask me.
If great Defense = hard to hit cleanly, then I would say Young Griffo, Corbett, Leonard, Tunney, Pep, Howard Davis jr, Ali, Jimmy Young, P. Whitaker, and W. Wright are in the conversation.