I’ll always recommend Exiles. It’s what happens when X-Men and Sliders have a baby. A ragtag group of B-to-C tier mutants solve problems and wreak havoc across the multiverse.
And fuck I love Morph!
My Go-To suggestion on the DC side is the James Robinson run on Starman.
I’m a little biased towards Batman comics, but I’m not sure where your interests lie or what you’ve read already.
I really liked the Court of Owls storyline. The conspiracy and intrigue was right up my alley. I liked how the story was told and what the story was.
As far as more standalone stories go, there’s always the classics: Year One, The Dark Knight Returns, and The Killing Joke. If you ask about Batman comics, these are always going to be on someone’s list
I’d like to add The Long Hallowe’en. That was some good storytelling.
Court of Owls
I second this rec. It was a fantastic storyline.
What kind of thing do you like? What have you already read?
The classics:
DC: Watchmen, Kingdom Come, Frank Miller’ Dark Knight, Wolfman/Perez’s Teen Titans and a lot of the Vertigo line or the comics that led into it: Alan Moore’s Swamp Thing, Grant Morrison’s Animal Man, Neil Gaiman’s Sandman, Hellblazer, GM’s Invisibles, etc.
Marvel: early Lee/Kirby/Ditko (FF, Spider-Man, Doctor Strange, etc), Chris Claremont’s X-Men run, Frank Miller’s Daredevil and Elektra, Alan Moore’s Captain Britain then on to Claremont’s Excalibur, etc.
Modern stuff:
DC: GM’s Batman run and All-Star Superman, Scott Snyder’s Batman, etc.
Marvel: Mark Millar’s The Ultimates (inspired The Avengers in the MCU), GM’s X-Men run, Al Ewing’s Immortal Hulk, modern Guardians of the Galaxy (starting with the Annihilation event, then Abnett/Lanning’s run on GotG, Nova, etc and to Cates/Ewing), etc
DC adjacent (they now own them): Ellis through Millar’s The Authority (which the Ultimates and GM’s X-Men grew from), V for Vendetta, etc.
Marvel adjacent (they now own them): Marvelman/Miracleman.
Tacking on here after X-Men were mentioned:
The adjectiveless X-Men of the early 90’s was a great volume, beginning with a stellar run sketched by Jim Lee.
Old Man Logan’s really good for Marvel
On the DC Side, I enjoyed the hell out of the Forever Evil event and its various offshoots.
I’d also recommend checking out the indie titles put out by Boom and Image. There’s a lot of great work being made in the non-superhero stories.