Pound for pound, I don't know if there was ever a better fighter than either Clark Gillies or Terry O'Reilly. I'd definitely included both of them amongst the best fighters ever, up there with Bob Probert and John Ferguson. And the best thing among all 4 of these guys - they were all very good hockey players too.
In the late 70s and early 80s, these two waged many of their own battles in a bitter war between the Islanders and Bruins. Over at Hockey's Tough Guys, I've found video of 5 different fights between the two. The score card? 2 wins each, 2 losses each, and 1 draw. That is of course, if you're not counting O'Reilly's DQ in fight 5!