DC proper, in no particular order...
All Star Superman
Seven Soldiers + Final Crisis (I see them as one big thing)
Dark Knight Strikes Again
Azzarello + Chiang's Wonder Woman
I'd have to think harder for a more expansive thread, but...
Sandman: Endless Nights/Sandman Overture
...would all be included.
EDIT: Lot of Morrison on the list. I think lately I've started to come off him just a little bit, but his work in the DCU has still be seminal for me, always interesting and energetic (in no small part due to the collaborators he gets).
By the time we come to Multiversity his superhero work often has the feel of regurgitation, but when it hits its highs it's legitimately brilliant(Pax), packed with ideas (Secret Society,) or very fun without being simple (ThunderWorld).
I think, with no constraints, Tom King has the potential to create the next big thing as he did with Vision, his Animal Man or Swamp Thing. I hope so, anyway.