Holyfield by far. He's a professional headbutter. I'm convinced he practices in the gym where to put his head when a fighter is coming in because he always seems to have his head in the right place at the right time. It's way too much of a coincidence.
Holyfield is not only a legendary boxer, he's a legendary headbutter. If he wasn't such a likable guy who gave so much to the sport, then I think he would get 10 times more criticism for it. Hopkins has always been extremely sneaky with dirty tactics and I expect him to try to use headbutts against Calzaghe. Whether you think it was intentional or not, it worked against southpaw Winky in his last fight and he got away with it.