I picked Bernard Hopkins, not based on chin, but his ring generalmanship and overall movement. Very hard to catch cleanly, or be effective against.
Great chin, but he can be caught easily when he plants his feet and starts to unload with both hands. This will eventually lead to his demise, against a noted puncher. If he were to let someone like Pac catch him with the punches that Soto was, he will get hurt regardless of how good his chin is IMO.
He would definitely be up there. He is a guy who has fought noticeably bigger fighters in Hopkins and Taylor and he was the one that kept coming forward and eating punches. He's a warrior. He also fought Vargas in a tough fight and never backed down. The guy's chin is underrated and his defense is incredible.