Kirkland is my first choice. He got flattened by a Jab from a Feather Fisted Japanese guy who had something like 7 KOs in thirty fights. Other acceptable answers are Judah, Roy Jones, Enzo Macaroni, Herbie Hide, Fraudley "A-Farce" Harrison, and Khan. They all have awful Chins.
Right now, it's Roy Jones Jr. miles ahead of Wlad. RJJ, however, is the only example I can give of a fighter who was undoubtebly the #1 P4P fighter of his era to climb to the top of the "glass jaw" rankings later in his career. Keeping with your current or past champion criteria, anyway.
Actually, I reckon he has been down around 4-5 times as a champion. Against Sanders and 3 times vs. Peter. As far as I recall he didn't go down vs Brewster (and anyway, he wasn't a champ for that fight). Apart from the Sanders fight, his losses were more due to gassing than due to his chin, so I wouldn't rate him anywhere the weakest. Not a good chin by any stretch of the imagination, but not horrible either.
I'd like to question the premise of some of the posts: getting knocked down isn't neccessarily the same as having a weak chin. Could be a balance issue, could be because the guy is reckless or exhausted. Getting stopped often by seemingly weak punches is what counts.
The thing about Wladimir klitschko is that he hasn't the best chin but still dominates. Quite astonishing really but anyways. Roy jones jr probably hasn't a good chin but in hes prime and when he was jucing he relyed alot on his athletic abillity wich has deteriated alot since then. Didn't the acuse Lennox of having a glass jaw?