The entire Sinestro Corps War story probably the best comic book event of the past 20 years. In some ways it's become underrated though because the sequel wasn't nearly as good and the Lantern franchise has fallen off hard since then.
The Death and Return of Superman is always going to hold a special place in my heart.
Cable & Deadpool "If Looks could kill" and "The Burnt Offering" are still the two best stories starring either of those characters.
Black Panther "The Client" is probably that characters definitive story and "Enemy of the State II" is an underrated classic.
Geoff Johns Legion story that started in Justice League and Justice Society, went over to action comics, and finished in Legion of 3 worlds....
Even if you're not a fan of DC stuff you NEED to read that entire epic ass story.
Daredevil from the time Bendis took over till the end of Brubakers run is amazing.
The entirety of Planetary
The first couple of arcs from Irredeemable.
The entire Prison arc from the Walking Dead.
Way too much dope shyt to name to be honest.