The Ultimates. The original run. Say what you want about the Ultimate universe and how it sucked, the Ultimates was good. I liked how it reimagined the classic Avengers for modern day, I liked the story, I liked some of the characters. Except for Hank Pym because he sucks.
Amazing Spider-Man #121-122. The Night Gwen Stacy Died. A shocking and tragic story that I love to this very day.
Kraven's Last Hunt was a great Spidey storyline that pretty much made Kraven my favorite Spider-Man villain. Beautifully done, and totally entertaining.
Alan Moore's Watchmen reinvented the comic industry. A great mini-series that got adapted into a mediocre film. Seriously, if you're interested in Watchmen, read the mini-series instead of watching the film.
Joe Kelly's original run on Deadpool. Before Deadpool became an obnoxious, overrated, barely-even-an-actual-character-anymore internet meme, Joe Kelly did a great job writing a comic book about him.