It sounds like there might be a bit of a name mix-up! While there isn't a character named "Mr. Farnworth" in Citizen Kane, you are probably thinking of Jim Gettys (played by Ray Collins), the political boss who acts as the movie's primary "gangster" figure.
If you’re referring to the best "gangster" of all time in the context of this film's world or its actors, here is the breakdown of who actually fits that description:
Boss Jim Gettys: He is the shrewd political machine leader who expertly takes down Kane. He doesn't use guns; he uses a blackmail letter and a quiet confrontation in a hallway to destroy Kane's election chances. He is the ultimate "clean" gangster who knows exactly where the bodies are buried.
The "Gangster" Vibe: The way Gettys is filmed—often in shadows or looking down from balconies—gives him that menacing, untouchable quality you’d expect from a mob boss.
The Actor Richard Farnsworth: Interestingly, there is a legendary actor named Richard Farnsworth, but he was famous for being a stuntman and Western star (like the stagecoach robber in The Grey Fox) rather than a mobster in Citizen Kane.