Being a Giants/Yankees/Rangers/Knicks fan, I can tell you that sports fans in the Northeast are just a bunch of monsters. Sure, Raiders fans are rough, Pistons fans can riot, but nothing can compete to the triangle-of-vulgar-fans between Boston, New York and Philadelphia for sheer stupidity and aggression.
Philly Eagles fans started cheering when Michael Irvin of the Cowboys was injured in such a way it looked like he might have broken his neck (I have no love for Irvin or the Cowboys, but c'mon!). Phillies fans threw batteries at J.D. Drew, the guy who refused to play for Philadelphia, when he returned in a Cardinals uniform.
At a game in Fenway, I saw an incredibly drunk Red Sox fan pick on a 12 year old boy for about an hour, cursing, yelling, etc. I thought maybe the kid was a Yankee fan getting the treatment, but when he turned around eventually, I saw he was wearing a Red Sox cap! I was wearing my Yankee cap and received a bit of the treatment too, heh.
In NY, I personally got into an altercation at Yankee Stadium when a group of highly inebriated Mets fans (yankees vs. mets game...add alcohol, stir vigorously) started yelling up racial slurs and epithets at me. I don't know what I had done to bring attention to myself. Anywho, I had a video camera I borrowed from work at the time and I decided to record the jackasses who astonishingly went very quiet. Until, that is, one of them got up and came up to my row demanding the camera from me. Now, this was a sony digital camera worth about $1200, uninsured and owned by my place of employment. He kept taking swipes at the camera and I warned him off. Since he wouldn't stop, I kicked him really hard in the sternum/chest area twice. This finally made him back off muttering while clutching his chest region. Stupid, stupid, stupid.
Oddly enough, most of the Mets fans in the area who were razzing me before hand were otherwise pretty cool. Just a couple of drunk, bridge n' tunnel kids ruined it.