I've seen 4!
In Issue 241 (July 2009) of EMPIRE magazine, readers found a list titled "The 20 Greatest Gangster Movies you've never seen ...probably"
1. The Roaring Twenties (1939) Passed | 106 min | Crime, Drama, Film-Noir Three men attempt to make a living in Prohibitionist America after returning home from fighting together in World War I. Director: Raoul Walsh | Stars: James Cagney, Humphrey Bogart, Priscilla Lane, Gladys George 2. Villain (1971) R | 98 min | Crime, Drama, Thriller In 1970s London, Scotland Yard orchestrates the downfall of mob boss Vic Dakin after he crosses the line by blackmailing Members of Parliament. Director: Michael Tuchner | Stars: Richard Burton, Ian McShane, Nigel Davenport, Donald Sinden 3. A Bittersweet Life (2005) Not Rated | 119 min | Action, Crime, Drama Things go wrong for a high ranking mobster when he doesn't proceed by his boss's orders. Director: Jee-woon Kim | Stars: Lee Byung-hun, Shin Min-a, Yeong-cheol Kim, Hwang Jung-min 4. 99 and 44/100% Dead! (1974) Passed | 98 min | Action, Adventure, Comedy Uncle Frank Kelly calls on Harry Crown to help him in a gang war. The war becomes personal when Harry's new girlfriend is kidnapped by Uncle Frank's enemy, Big Eddie. Director: John Frankenheimer | Stars: Richard Harris, Chuck Connors, Edmond O'Brien, Bradford Dillman 5. Tokyo Drifter (1966) Not Rated | 82 min | Action, Crime After his gang disbands, a yakuza enforcer looks forward to life outside of organized crime but soon must become a drifter after his old rivals attempt to assassinate him. Director: Seijun Suzuki | Stars: Tetsuya Watari, Chieko Matsubara, Hideaki Nitani, Tamio Kawaji 6. Gabriel Over the White House (1933) Passed | 86 min | Drama, Fantasy, Romance A political hack becomes President during the height of the Depression and undergoes a metamorphosis into an incorruptible statesman after a near-fatal accident. Director: Gregory La Cava | Stars: Walter Huston, Karen Morley, Franchot Tone, Arthur Byron 7. The St. Valentine's Day Massacre (1967) Not Rated | 100 min | Crime, Drama, History Al Capone's Valentine's Day surprise for the rival Bugs Moran gang in 1929 Chicago. Director: Roger Corman | Stars: Jason Robards, George Segal, Ralph Meeker, Jean Hale 8. Underworld U.S.A. (1961) Not Rated | 99 min | Crime, Drama, Thriller A teenager who witnesses the murder of his father vows to exact revenge on the four mobsters involved in the killing. Director: Samuel Fuller | Stars: Cliff Robertson, Dolores Dorn, Beatrice Kay, Paul Dubov 9. The Violent Professionals (1973) R | 104 min | Crime, Drama, Thriller When his colleague is murdered by an organised criminal gang, a Milanese cop goes undercover to continue the investigation and single-handedly destroy the organization from within. Director: Sergio Martino | Stars: Luc Merenda, Richard Conte, Silvano Tranquilli, Carlo Alighiero 10. Kiss Tomorrow Goodbye (1950) Approved | 102 min | Crime, Film-Noir, Thriller Starting with a violent prison break, clever, ruthless Ralph Cotter corrupts everyone around him. Director: Gordon Douglas | Stars: James Cagney, Barbara Payton, Helena Carter, Ward Bond 11. Dillinger (1973) R | 107 min | Action, Biography, Crime John Dillinger and his gang go on a bank robbing spree across the midwest, but one G-Man is determined to bring him down. Director: John Milius | Stars: Warren Oates, Ben Johnson, Michelle Phillips, Cloris Leachman 12. The Grissom Gang (1971) R | 128 min | Crime, Drama Set in the Depression, a gang of half-witted small-time hoods led by Slim Grissom kidnap heiress Barbara Blandish and Slim proceeds to fall in love with her. Remake of the British film No ... See full summary » Director: Robert Aldrich | Stars: Kim Darby, Scott Wilson, Tony Musante, Robert Lansing 13. Pete Kelly's Blues (1955) Not Rated | 95 min | Crime, Drama, Music In 1927, a Kansas City, Missouri cornet player and his band perform nightly at a seedy speakeasy until a racketeer tries to extort them in exchange for protection. Director: Jack Webb | Stars: Jack Webb, Janet Leigh, Edmond O'Brien, Peggy Lee 14. Prime Cut (1972) R | 88 min | Action, Crime, Drama A vicious Kansas City slaughterhouse owner and his hick family are having a bloody "beef" with the Chicago crime syndicate over profits from their joint illegal operations. Top enforcer Nick Devlin is sent to straighten things out. Director: Michael Ritchie | Stars: Lee Marvin, Gene Hackman, Sissy Spacek, Angel
15. Monument Ave. (1998)
R | 93 min | Crime, Drama, Thriller
A small-time hoodlum is divided between his conscience and loyalty to his boss when his cousin is murdered.
Director: Ted Demme | Stars: Denis Leary, Ian Hart, Jason Barry, Lenny Clarke
16. The Funeral (1996)
R | 99 min | Crime, Drama
After the funeral of one of their own, a criminal family decides to embark on an emotionally unnerving journey in an attempt to exact bloody revenge.
Director: Abel Ferrara | Stars: Christopher Walken, Chris Penn, Annabella Sciorra, Isabella Rossellini
17. The Penalty (1920)
Passed | 90 min | Crime, Drama, Thriller
A deformed criminal mastermind plans to loot the city of San Francisco as well as revenge himself on the doctor who mistakenly amputated his legs.
Director: Wallace Worsley | Stars: Charles Clary, Doris Pawn, Jim Mason, Lon Chaney
18. Black Caesar (1973)
R | 87 min | Action, Crime, Drama
Raised in Harlem, Tommy Gibbs becomes a successful mob boss but he clashes with the rival Mafia and his old enemy, dirty cop McKinney.
Director: Larry Cohen | Stars: Fred Williamson, Gloria Hendry, Art Lund, D'Urville Martin
19. Borsalino (1970)
R | 125 min | Crime, Drama
During the 1930s, in Marseilles, France, two small time crooks work for local crime bosses until they decide to go into business for themselves.
Director: Jacques Deray | Stars: Jean-Paul Belmondo, Alain Delon, Catherine Rouvel, Françoise Christophe
20. A Better Tomorrow (1986)
Not Rated | 95 min | Action, Crime, Drama
A reforming ex-gangster tries to reconcile with his estranged policeman brother, but the ties to his former gang are difficult to break.
Director: John Woo | Stars: Lung Ti, Leslie Cheung, Chow Yun-Fat, Emily Chu