Mark Hunt is up there, although he maybe has only ever been hit hard in kickboxing, not mma. I've definitely seen some lower level guys take huge shots before, so it's strange to only single out the famous fighters. It's a huge asset for any fighter, one of those unusual variables that makes MMA more interesting than other sports.
Anderson Silva certainly doesn't have a glass jaw, but he also has somewhat rare skill of being good at seeing punches and being able to roll with them. It's a common skill in high level boxing. I've never seen Silva actually get hard like Roy has been hit, which is really a testament to Silva's skill.
People are suspicious of GSP's jaw, but the two times he went down were to strikes to the side of the head. He might be harder to put away with strikes to the jaw or the face, but we won't know unless someone actually lands a good shot to his face, which will probably happen if he ever fights Silva.

Where is chris leben ???

Shane Carwin was last to go a full fight w/ JDS

Mhhh Frankie Edgar! Helloooo?

Anderson Silva hasn't been hit enough (Chael Sonnen's pillow fist dont count) to see if hes got a good chin. He just evades all the strikes And where are Keith Jardine, Arlovski and Terry Martin???

No Anderson Silva? Wack!