Not sure about all time, but Robert Guererro is DEFINITELY a dirty fighter. I like that though.

Also, may he Rest in Peace, but Agapito Sanchez was the definition of dirty. He used headbutts, elbows, lowblows everything. His fight with Pacquiao is one of the worst I've ever seen really. That could have been called easily in the second round. And if not the second, definitely the third.