Mark hunt definitely has a cast iron chin - just don't tell that to Melvin Manhoef. Andrei Arlovski should be the poster boy for MMA glass jaws.
Big Nog has took some beatings. Guida in his last fight i thought that Sanchez kick would have killed him
I think that mma fighters get a bit of a bad call when it comes to the bad chin/glass jaw accusations. Boxers/kickboxers specialize in striking. They get hit on the chin more than any other contact athlete. It should come as no surprise that elite strikers have elite chins. There are varying degrees to how good the elite striker's chins are, but for them to make it near the top at their sport; you can bet their chins have years of battle testing. Another thing experienced strikers do is learn avoiding being hit flush. That's not to say that mma fighters know nothing about striking, or they all get hit flush and have no defense. There are some experienced strikers in mma. I speaking in general terms here. A lot mma fights end because guys are hit flush with tiny gloves. Arlovski may not have a granite chin, but there are guys that do that would not want to get hit the way he does. Arlovski has been ko'd, but they were by monster shots recently. I don't think GSP was ill prepared for Serra. I don't think GSP is china chinned either. I think Serra hit GSP with a flush shot. Chuck Liddell is getting old, and one can't expect him to be what he once was. I still wouldn't say his chin is that bad, because he has been knocked out by big shots. He reflexes may be going, but I doubt his punch resistance is much worse than its ever been. Even the Kimbo Slice ko is overblown. He was floored by a jab, true. The jab also struck while Slice was commited to throwing a big right hand. Slice did make two attempts to stand. Slice was not knocked out until repeated blows to the head after being downed. That was no one-punch knock out.