Many English fighters like Joe Calzaghe, Carl Froch, Chisora, Fury and such have a very bad habit of rabbit punching, generally because it appears to be legal to punch in the back of the head in England.
Guys like Fritzie Zivic and Sandy Saddler are FAR more dirty than anyone like say Hopkins or Holyfield. Hopkins and Holyfield might coincidentally put their head where you're most likely to run into it, or go below the belt when the ref can't see... But guys like Sandy Saddler and Zivic would throw their hands straight out, glove open, to try to thumb your eyes out. They were seriously trying to hurt you, it was nothing for one of them to try to dislocate one of your shoulders for example. Guys like Hopkins, Holyfield etc. don't belong in the same category as those guys. The only modern fighter I've seen who was as nasty as those guys is Andrew Golota, he threw combos to the nuts, headbutted HARD, threw fake punches, purposely missing to connect with the elbows etc. Tyson could be nasty as hell too.
Not based on just one or two fights but being consistently dirty in majority of their matches. Bernard Hopkins Andrew Golota Andre Ward Edison Miranda Roberto Duran (my all time favorite fighter, but I have to be honest)
Zivic Pedroza Hopkins to the point where he even will fake an injury or leap out a ring or put on an act. Bradley headbutts the **** out of oponents Margarito should have been banned for life
Most of the dirtiest fighters would have fought in obscure clubs and most of us would never have even heard of them. But what Luis Resto did to Billy Collins has to put him near the top of the list.