I think it's interesting that John Ferguson, Tiger Williams and Bob Probert keep popping up as the best. If you go back and look at those 3 careers there is a 2 year overlap from one "Best Enforcer" to the next. I went back and checked the numbers because I could make an argument for all 3 guys. All 3 played in an All Star Game, all 3 could score and fight and seemed to be able to distinguish between the right and wrong times to do so. Ferguson had the highest points per game but had the shortest NHL career. Williams had the most PIMs but had the longest career and Probert was probably the most feared guy of his time but I think that's because the game was changing and there weren't as many tough guys in the league during his career.
So after all of this heavy reading I still can't pick one. They all played in a different era and there are too many variables. I think they were all the best of their time, Ferguson passed the torch to Williams and Williams passed it to Probert and unfortunately I think the NHL has extingushed the flame and it won't be lit again.
Its tough to pick among those three, but I think they are certainly the top 3, there are some other solid choice out there (Schultz, Semenko, Nilan et al.) and probably some nice ones pre-Ferguson (though its tough to say not having much first hand accounts from people to tell me about them).
Though for the toughest player all time, its still Stan Jonathan, I'm not sure if he should be consider a true enforcer though, I have watched a lot of old Bruin tapes in my time (my step-father was a big fan) and the dude was just undeniable, he could take anything.