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!
If beltholders are included, Hide and Bruce Seldon are good candidates.