All the suggestions so far are good but it is hard to rank them. It is easy to move Margo down the list since he just got KO'ed but you have to look at the manner of it. His ability to take a hard flush shot is just as good as anyone else's that has been mentioned but he just couldn't take the sustained attack of solid shots dished out on him and many that have been mentioned have never took as many flush shots as Margo did during the Mosley fight. It's easy to say that Hopkin's and AA have better chins but neither of them have ever taken the amount of shots Margo did as they are hard to hit. There are really two ways to rate a chin. How much of a single punch it can take and how much of a sustained beating it can take. All people mentioned have shown that they can take a good, hard, flush shot, but people like Margo, Froch, Peter have shown that their whiskers can take a sustained beating. Fighters like Hopkins have not but then again that is a good thing. Why prove you can take a beating when you can just be a defensive genious instead.
Toney Abraham Hopkins They are my clear top 3, vitali, peter, froch, calzaghe, mosely and margo all deserve mentions, as does mayweather if he unretires