I think with Rigo you've gotta take his amateur achievements into account as well. His skill's still a great spectacle to watch, hope he does a lot more in the pro ranks. For the record, I've got Toney as my favourite slickster.
Khan. No one can become ever more elusive by back peddling than when Khan is in trouble. You're best bet is to catch him by surprise because if not, he will run all over that ring, and then some to avoid punches.
Sugar Ray Leonard was IMO the best boxer/fighter that I've ever seen. His performance against a prime Duran in the No Mas fight was sublime and unmatched.
Pernell Whitaker would stand directly in front of other good boxers and make them miss, all the while, landing some counter shots. Incredible feat!
nicolino locche. I saw clips of him and he was definitely the model for the defensive style of Floyd Sr and Jr.
Besides the same mentions you typically see on boxing forums when this question is asked--guys like Pep (greatest defender ever, in my opinion), Mayweather, Toney, Benitez, Locche, Hopkins, etc.--there were plenty of great guys whose names should be right up there with them. Maxie Rosenbloom, Ivan Calderon, Young Griffo, Joe Gans, Tommy Loughran, Happy Lora, George Benton, Benny Leonard, Miguel Canto, Freddie Welsh, Jim Driscoll, Gene Tunney, James J. Corbett, Charley Burley, Jack Johnson, Chris Byrd, Harry Greb, Mike Gibbons, Melchor Cob Castro, Holman Williams, and a number of others. I think Duran, Napoles, and some others are underrated. Lomachenko, Ward, Lara, Rigondeaux, Vazquez, and Cuenca are some really solid modern defenders.