Hmmm, a little tough to say, but I would think Loma. Pushed himself by taking on more weight classes and challenges and was ring champ as well. GGG never been ring champ or undisputed from what I can remember. He also passed up challenges at 168.
Both are sraight in the hall of fame, and I believe when you factor in Lomas amateur career he'll be classsed as an all time great. Plus he's not done yet, I expect Loma to move back to super feather and clean up.