There have been so many individual dirty players over the years and most have already been mentioned above. For collective dirty players though, the worst I've seen are:
Leeds United throughout the 1960's & 70's - cynical to a man.
Racing Club (Buenos Aires) v Celtic - 1967 World Club Championship - Celtic got kicked to pieces in the first 2-games but decided to get their retaliation in first in the decider (which ultimately cost them the trophy).
Estudiantes (La Plata) v Man Utd - 1968 World Club Championship - Veron's old man played for these thugs.
Estudiantes (La Plata) v AC Milan - 1969 World Club Championship - completely outclassed by Milan, the Argentinians literally went nuts at their place. The 2nd Leg in La Plata was the worst (but most entertaining!) thuggery I've ever seen on a football pitch. Their goalie got so out of hand, he ended up going to jail.
For the title of dirtiest player in any sport, I've got to give a mention to the Philadelphia Flyers henchman, Dave "The Hammer" Schultz. I can still remember the mid-70's when the BBC used to show the Stanley Cup Finals on Grandstand with Alan Weekes commentating - absolutely brutal bloke but fantastic entertainment.