I think he just means fighters who fight good fighters not so much what they do in the ring.

However, I think more impressive then anyone are fighters like Pacquiao, Morales, De La Hoya Barrera, Taylor (so far), and others who are CHAMPIONS yet still fight top notch opposition. They are the ones in the position to lose and not gain, and still face the very best fighters. That is much more meaningful then someone like Zab who is always the challenger when he faces good opposition.