Whatever Happened to the Man of Tomorrow? Dare I say the quintessential Superman story.
The Proteus arc of Uncanny X-Men, while not as popularly know as Dark Phoenix or Days of Future Past, the Proteus issues are a trippy and engaging story from the classic Claremont/Byrne era of X-Men.
I have a real soft-spot for Morrison’s new X-Men since that (and the X-Men Evolution tv show) was my introduction to the characters. The art won’t be everyone’s cup of tea, but it has interesting ideas, excellent plotting and character work. In particular I loved the characterization of Jean, Logan and Emma in this run.
The Death and Return of Superman. It’s the story line that got me into comics. I still remember getting excited each week when the next book in the storyline came out. All Star Superman is a close second.
But if you count Vertigo as part of DC, then my answer is Sandman.
DC:
Kingdom Come
Superman for All Seasons
Superman: Red Son
Marvel:
Spider-Man: Blue
Marvel: 1602
Taskmaster: Unthinkable
Special shout out to the X-Statix omnibus. I bought it knowing literally nothing about the characters. It’s got the dimensions of a Manhattan phonebook and I read the whole damn thing and loved it. One of the wildest and weirdest things I’ve ever read set in the Marvel universe. 10 out 10, no notes.
The Magik limited series: I just thought it was so cool - her powers, everything she goes through, etc.
Avengers vs. JLA: Just going to sum this one up with one of the coolest scenes in comics IMHO -
Age of apocalypse ftw