Seconded. Leonard is a strong second, then I've got Mayweather, Pacquiao and Jones (no order) to fill out the top 5.
probably some forgotten overlooked Contender or Champion that gets missed owing to Achievements Based Lists... it is a hard one without thinking and looking back over Resources, but maybe JCC or Roberto or PBF. But I'll bet some of the more schooled Boxing Nerds will remind us of a just as viable a name or two.
JCC, Duran, Mayweather, and Leonard are in the running. Do we want greatest active fighter? Might just be Naoya Inoue.
Looking down my p4p ATG list: SRR Greb Langford Armstrong Charles Fitz Duran Duran is the first fighter I come to still alive, so he gets my vote too.