Alexis Arguello, definitely.Based on what I've read from several of his contemporaries and ex-opponents, Greg Page is up there as well.
I'll just comment on the fighters I have met - Jersey Joe Walcott, Rocky Graziano, Floyd Patterson, Joe Frazier, George Foreman, Shannon Briggs, and Chuck Wepner were all very pleasant, friendly, and approachable. Floyd Patterson engaged in a conversation as did Chuck Wepner. Also, to my surprise, Roberto Duran was quite friendly when I met him at the Larry Holmes training center in the early 80s.
Manny Pacquiao, Nonito Donaire, Andre Ward, W Klitschko, and Lucian Bute seem very chill people from interviews I've seen. Rocky Marciano and Ricardo Lopez deserve mentioning.:bbb
Best person I ever met anywhere around boxing was a journeyman fighter named Gary Gamble who fought Boza Edwards on Sportsworld once upon a time. Just a wonderful guy who was always willing to help the kids and the no-hopers at the Indy PAL with encouragement and tips. Same with Champ Chaney, Marvin Johnson's trainer, who was like a pastor in how he carried himself.