Lennox by a whisker. Better mental reaction to getting hurt as well. Wlad's chin wasn't actually that bad; it tended to take a lot of consecutive hots to put him down, and I don't recall any time he was completely knocked unconscious (though Sanders came close). HIs mental reaction was what used to let him down. As for Lennox, he actually had a pretty good chin as far as champions go. The only time he got truly flatlined was against Rahman and that was a monster shot right on the button. Do a side by side with the OH right that Shavers landed on Holmes and using the video footage alone tell me which looked the harder punch. It's difficult to do.
By the way, has anyone felt how solid and indestructible a good lump of glass actually is? If a fighter's chin was literally made of glass they'd be breaking knuckles.
He was no Oliver McCall, but his chin was perfectly respectable even at elite heavy level, loads better than that of a number of peers or near-peers such as Wlad, Frank Bruno, Bruce Seldon, etc. I agree with the poster who gave it 7/10 at world heavy level.
Lewis's chin isn't glass when compared to tomato cans like the well-named Eric Crumble, but among top tier fighters it stands out as among the more fragile. Fighters with good chins don't get taken out in the early rounds by second raters like McCall and Rahman in the heart of their careers. Period.
Better than Bruno's, not totally glass but not totally iron either. It was possible to end a fight against him with one punch.