I'm mostly a silver/bronze age marvel stan, so there's a lot of good DC stuff I haven't read. I liked Watchmen, the Dark Knight Returns, Year One, and All-star Superman though. I'm open to suggestions for GOAT level DC storylines
Grant Morrison's Animal Man. Jeff Lemire's run was really good too
Alan Moore's Swamp Thing run. Scott Snyder's run was good and tied into Lemire's Animal Man.
52 the weekly series was surprisingly solid the whole way through.
Days of Vengeance
Court of Owls
Knightfall
I, Vampire (Nu52) might not be for everybody since it isn't a superhero book.
Jonah Hex (the pre 52 run)
There was a Spectre book that launch after Infinite Crisis that had a Dr. 13 backup. Both stories were really good
I read that the Vertigo Haunted Tank was really good but I haven't gotten around to it yet
Sandman Mystery Theatre
Suicide Squad Trial by Fire
Superman Whatever Happened to the Man of Tomorrow?
Batman Long Halloween
Superman For the Man That Has Everything
There was a Steel elseworlds story that was set during slavery that I remember really liking as a kid. Not sure how it still holds up
Those are just some. They got a lot of little stories that are underrated