This being the most controversial film depends, of course, on what you mean by it: For a pornographic visualization of fascism I think Posaloni's Salo is 10x more unwatchable (https://en.wikipedia.org/wiki/Salò,_or_the_120_Days_of_Sodom). There are also more recent exploitation films that offer much more shock value. I think the issue here was the Nazi connection.
