in my opinion the best acted villain is Lil'Ze by Leandro Firmino da Rosa in City of God. I don't know if this is actually a mob movie but the acting is so amazing that I voted for it. maybe is not even the acting, but the actor is so perfect for the role that it turned out to be the best acted villain in my opinion

obviously, honorable mention for Joe Pesci and Al Lettieri too. I also like Lee Strasberg as Hyman Roth. is a different kind of villain, and is also perfect played