Among belt holders? Probably Herbie Hide There are always guys that get hit with a punch and go down for the count just to collect their paycheck. I don't count them since there have been so many in history.
Herbie hide without doubt, then again some othe rfighters who are bums are obviously worse but at title level it has to be hide.
Linear champs?? You gotta put Lennox Lewis in the mix here - the only HW champ taken out in the early rounds twice by ordinary contenders when he held the title! Of course, Floyd Patterson should be in the discussion here, too - he was taken out or knocked down even more frequently, although usually against better fighters. But probably THE worst chin on a linear HW champ belongs to Hasim Rahman. Now that's some serious Grade A China right there! :nod If beltholders are included, Hide and Bruce Seldon are good candidates.