Roma Ultras are possibly the most notorious in Italy, I think their (supposed) political allegiance is to the far left though (not sure tbh), with Lazio having the reputation for their Ultras being Fascist.
Roma have a far right following as well, the whole left-right thing between them and Lazio isn't especially true (I know a couple of Lazio fans who are definetly not facists!) But on the whole i would say Lazio have the biggest facist following in Italy. Great article here btw.
the nappyrippers ain't having much luck abroad it seems.