Winstone? Juan Manuel Marquez clearly has a vulnerable chin but maybe he doesn't qualify because his recovery powers are so amazing. Steve Luevano had amazing skills but his chin was fragile.
I'd have to say Floyd Patterson. Technically he was an incredibly skillful fighter but his chin let him down many o' times.
The obvious ones that get brought up all the time here, Jones Jr and Wlad. Other suggestions would be Hearns... and that's all I got off the top of my head. Good topic though, I'll be back. A Mexican with a bad chin would be interesting, considering their culture.
I would agree that JMM is somewhat chinny, considering he's been knocked down AT LEAST 5 times in his career (4 by Pacquiao, 1 by Mayweather)
I don't know if I agree that Roy Jones' chin was weak. I think his punch resistance diminished with age, and in most cases, it still took a pretty big hitter to finish him even then. Meldrick Taylor was a very talented boxer who was on the verge of acheiving greatness, and who's chin I think was highly questionable. Roger Mayweather didn't always have the best set of whiskers either.
Wilfred Benitez. Thomas Hearns. Felix Trinidad. Nigel Benn. Dont consider them to have glass chins but they were def shaky.
Terry Norris springs to mind,hearns didnt have a glass jaw,he was vunerable at times against big punchers but didnt have a bad chin..