good list
I'd add on
Aaron Pryor
Henry Armstrong
Felix "Tito" Trinidad
Holy shit!! how could i forget Pryor.
Trinidad is a good call to.
I gotta say i think Frazier is far and away the outstanding candidate for the MOST relentless. He was all over you like a rash from bell to bell.
Joe Calzaghe
This is the problem. We all define things differently.
Pac, Hatton, Gatti were/are relentless. The OP mentioned Frazier, Duran and JCC so I'm assume relentless PLUS good enough ability to get past great boxers. There is also an awesomeness factor a fear factor involved and Calzaghe does not have that.
He has more power than his distractors give him credit for. Slaps don't knock out Roy Jones or keep Hopkins covered up for 12 rounds but I wouldn't call him relentless. Active, busy but not relentless.