First, Inter fans are not violent if you compare with other teams but I don't know very much about Italian hooligans. Arsenal fans was very violent but now that their tickets cost 50£ or more you don't see many hooligans around Highbury anymore. What the Barcelona fans did was not anything extreme, Figo did something horrible and one should be glad that the Ultras didn't do something worse. Every time I watch a game things are thrown in on the pitch, for example billiard balls, darts, bananas, stones, bottles, bengals etc. and those things often have some sort of target.
So which country has the worst fans?
That is a very simple question, Poland. I can't understand why nobody has mentioned them.
The polish steroid hooligans don't have any limits at all. They often use weapons and always kick laying men. There's much to say about polish hooligans like the fight before Poland-England.
The Galatasaray fans are violent but all easteuropean teams are violent.
England was very famous for their hooligans but along with the high ticketprices the hooligans disappear. The first one you come to think of is Millwall but they have really taken care of the problem after the incident after the Birmingham playoff game last year.
The ones I have most respect for today is Cardiff, Burnley and Portsmouth. Also Manchester United deserves a lot of respect, they have big numbers but they aren't organised.
If I had written this post ten years ago it would have looked very different, much has been done since then. Nowadays it is easier to have control of the hooligans as you can see who's a hooligan just by looking at his/(hers) clothes.