No mention of Julio Cesar Chavez and he is imo by far the best parrier that I've ever seen. He made it seem effortless the way he'd pick shots off with his gloves. Like a man blocking the shots of a child. Chavez was the best at it, none of those mentioned in this thread come close at it.
While JCC was great at it, I believe his subtle movements are what made his defense great. While he may have had incredible parrying skills, it was actually one of ODLH's greatest defensive move.