When you get KOed by a very heavy handed fighter like say a Sonny Liston, Julian Jackson, Kostya Tszyu, Ray Mercer, or Big George doesn't mean you have a weak chin. It means you made the mistake of being hit clean by a very solid puncher.
As far as top contenders or Champions? Ill go with Augie Sanchez,, had the whole package the only reason he never developed into an elite champion was because his chin was china. As for elite or HOFamers? Of the top ill go with Terry Norris.
I agree: most iron chinned guys don't get hit flush repeatedly, because they also have a good defense. Tommy Morrison on the other hand had a terrible defense, that's why he got hit flush all the time - i.e. he got knocked out. A weak chin is, if you get buckled or knocked down from a nothing punch or a jab.