Vitali comes to mind first b/c he's never been knocked down or out. Tua, Holyfield, McCall, and Ali had amazing chins as well.
That's because he always quit, or got the doctor to quit on his behalf when he was in danger of being KO'd. VITLAY doesn't belong on this list because he's only taken a couple of big shots in his entire career, and both times he wobbled badly - in fact in one of them he hit the deck only for the idiot referee to rule it a slip. Oliver McCall, George Chuvalo, Muhammad Ali are good names for this one, as is Larry Holmes, and George Foreman. Compare VITLAYS comp to these guys and you'll see why he just doesn't belong.
Max Baer. In 84 fights, he was stopped for a ten count ONCE, against ATG Joe Louis - and even then, he was completely conscious on the canvas, and basically chose not to get up. The dude had a IRON chin, one of the best in HW history.