What is The Greatest Marvel Comics Story Of All Time?

Marvel's movies are generally pretty great, and Agents of SHIELD is becoming downright entertaining. But they've still barely scratched the surface of the wealth of material that Marvel has run in the comics. What's the absolute best Marvel Comics storyline of all time?

Please include a picture or cover for your chosen storyline. And please mention the name of the comic, the name of the storyline or issue number, and why it's so great.

Top image: Avengers, "Ultron Unlimited" by Kurt Busiek and George Perez. The evil robot that Hank Pym created finally pushes the Marvel heroes to their breaking point —and they don't break.