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.
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.