1. The Sandman by Neil Gaiman
While Watchmen was originally published as 12 issues, The Sandman by Neil Gaiman is much longer, originally running at 75 issues. The Sandman is a great comic series to start reading if you're looking for something on the longer side. It's worth noting that Neil Gaiman did contribute a bit towards Watchmen; whenever Alan Moore had trouble finding a quote for one of Watchmen's 12 issues, he asked Gaiman to help him. The series has been compiled into a few formats, which makes jumping into the series quite easy. Like Watchmen, The Sandman is notable for showing the potential of comics as a legitimate storytelling medium.
The Sandman is about Dream of the Endless, who represents dreams themselves. Despite the fact that it was originally marketed as a "horror" comic linked to the wider DC universe, The Sandman quickly spun off into its own world, filled with fantasy and myth.
The cheapest physical collection of The Sandman is a set of 10 paperback volumes, which has been recolored since the original release to better reflect what Gaiman intended. However, if you're more into premium formats, though, The Sandman has been released in an array of different extra-large hardcover formats that would make any collector happy.
2. Batman: The Dark Knight Returns by Mark Millar
The Dark Knight Returns was published around the same time as Watchmen and explored some similar themes. They both follow the path of an imperfect "superhero" who has returned to vigilante action despite government bans. However, in The Dark Knight Returns, that imperfect superhero is none other than Batman.
The Dark Knight Returns and Watchmen are notable for popularizing the term "graphic novel" and adding legitimacy to a medium that was previously seen as something only meant for children.
On the other hand, The Dark Knight Returns and Watchmen share one other piece of legacy that may not be as positive . . . the universally hated font called "Comic Sans" is based on the hand-lettered text used in both Watchmen and The Dark Knight Returns.
The Dark Knight Returns can be purchased as a single, collected volume nowadays.
3. Saga of the Swamp Thing by Alan Moore
Why look farther than the same author when looking for content similar to Watchmen?
Alan Moore took over Swamp Thing about 20 issues into its second series and turned a struggling series into a hit by killing off most of the characters in a single issue and completely recreating the true nature of the protagonist, the titular Swamp Thing.
Alan Moore's Swamp Thing run is considered to be the beginning of a wave of comics that relied more on storytelling than action, eventually resulting in the creation of a new DC imprint called "Vertigo."
For extra points, Saga of the Swamp Thing takes place in the same continuity as Neil Gaiman's Sandman, which also made its way onto this list.
Saga of the Swamp Thing has recently been collected into six volumes.
4. Doom Patrol by Grant Morrison
Like Saga of the Swamp Thing and The Sandman, Doom Patrol by Grant Morrison was also published by DC's Vertigo imprint. Doom Patrol examines a team of oddball superheroes who often suffer more from their powers than they benefit.
Grant Morrison's entire Doom Patrol run was recently collected into three large paperback volumes, which makes it easy to get your hands on the entire story.
5. Kingdom Come by Mark Waid
Kingdom Come is often seen as a '90s response to the grittier graphic novels from the previous decade, like Watchmen and The Dark Knight Returns.
Fittingly, it abandons the dark inked lines of Watchmen and replaces it with beautiful painted art by Alex Ross.
Although it was originally released in the form of four separate issues, it has since been collected in a single volume, with additional pages added in to further flesh out the story.