Well, 'Batman: Year One' is definitely among the greatest. It tells the origin story of Batman in a really engaging way. Also, 'Kingdom Come' is great. It shows a future version of the DC Universe where the old heroes are forced to come out of retirement to deal with a new generation of more violent heroes.
Some of the greatest DC Comics graphic novels include 'The Dark Knight Returns' by Frank Miller. It redefined Batman in a darker and more gritty way, influencing the character's portrayal for years to come. Another great one is 'Watchmen' by Alan Moore, which is a complex and thought - provoking deconstruction of the superhero genre.
I think 'The Killing Joke' is a great one. It delves deep into the relationship between Batman and the Joker. Then there's 'Sandman' which, while it has elements that are more on the supernatural side, is a masterful work that has influenced a lot of other graphic novels. 'Green Lantern: Rebirth' is also very good as it revitalized the Green Lantern franchise.
In the realm of 'marvel dc greatest graphic novels', DC has 'Batman: Year One'. It's a great origin story for Batman, showing his early days in a gritty way. Marvel offers 'The Infinity Gauntlet'. This one is full of epic battles and showcases many of Marvel's most powerful characters. Another DC gem is 'Green Lantern: Rebirth' which revitalized the Green Lantern franchise.
You can start by asking in local comic book stores. The staff there are often big fans and can recommend some of the greatest DC Comics graphic novels. Also, online comic book communities are a great resource. People there are always discussing and sharing their favorite reads.
If you're a beginner, I'd recommend 'X - Men: Days of Future Past' from Marvel. It has an exciting plot with time - travel and a great cast of characters. From DC, 'Batman: The Long Halloween' is a great choice. It's a detective - style story set in Gotham. Also, Marvel's 'Daredevil: Born Again' is a good introduction to the character of Daredevil.
Some of the great ones include 'The Dark Knight Returns' which redefined Batman in a darker, more gritty way. 'Batman: Year One' that tells his origin story superbly. And 'Arkham Asylum: A Serious House on Serious Earth' for its psychological exploration of the characters.
Some of the greatest superhero graphic novels include 'X - Men: God Loves, Man Kills' which delved deep into the themes of bigotry and prejudice against mutants. It has great art and a strong story that still resonates today. Then there's 'Kingdom Come' which explores a future where superheroes have lost their way. The story is complex and the art is beautiful. 'Saga' is also a great one that combines elements of science - fiction and superhero - like characters in a very creative and engaging way.
One of the greatest is 'All - Star Superman'. It beautifully captures the essence of Superman with its engaging story and stunning art. It shows Superman's selfless heroism in the face of his impending doom.
Some of the greatest include 'The Dark Phoenix Saga'. It's a classic that showcases the X - Men's struggle with Jean Grey's transformation into the Dark Phoenix. Another great one is 'Civil War', which explores the ideological split among superheroes. And 'Watchmen' is also often considered among the best. It deconstructs the superhero concept in a very thought - provoking way.
'Superman: Red Son' is also among the greats. In this one, Superman's rocket lands in the Soviet Union instead of the United States. This alternate take on his story explores how his upbringing in a different environment would shape him and the world around him. It's a thought - provoking look at the character.