Definitely 'Sin City'. It's a classic. The way it portrays the seedy underbelly of Basin City and its gangsters is iconic. The contrast between light and dark in the art really emphasizes the noir feel. Another is '100 Bullets'. It's full of twists and turns with its gangster - inspired characters and complex plots. You can't put it down once you start. And 'The Boys' is also a great one. It's not your typical gangster story, but the way it shows the corrupt side of the superhero world with gangster - like elements is really engaging.
I'd say 'Road to Perdition' is a must - read. It has a heart - wrenching story about a father - son relationship set against the backdrop of gangster life. The art is simply stunning, making the world of the story come alive. 'Scalped' is also essential. It takes you into a world you might not be familiar with - the Native American reservation and its gangster - like problems. It's a deep and thought - provoking read. 'A History of Violence' is also a great choice. It makes you think about how violence and gangster - like behavior can be intertwined with normal life.