its pretty obvious some people can just take a better punch than others but i believe there are other factors that apply to punch resistance such where it lands and if the punch is seen coming. i don't believe anybody is uncrackable but some people just have titanium in their jaws as well as extreme mental fortitude which i believe plays a big part in taking a punch. An iron will can contribute to an iron chin.
If you are a youngster and get KO'd, but as you get older you get hit with powerful shots and don't get KO'd, it points to a mandible not being at full strength till your early to mid 20's. However some Glass Jawed Frauds have no chin at any age.
Hmm, I really don't agree with what you said about Shields. However, please don't overlook their skills with their chin. Sometimes they won't have a good chin and there is really nothing to do about it. I would give Shields a moderate chin, not iron or glass. I'd replace Cruz and drop him to Shields rank with Andrei Arlovksi. :think
Just an opinion, unless you are going to treat me like **** just because I am a "newbie" in this forum. Not saying that my rankings are right, just kind of confused about some of the rankings.