6. Ultron Unlimited (Avengers #19-22, 1999)
Kurt Busiek and George Perez’s run on Avengers during the late ‘90s was the best thing to come out of Marvel during that decade. They combined the classic Avengers style of storytelling with the more sophisticated plots being done at the time, which resulted in one of our favorite incarnations of Earth’s Mightiest Heroes yet. And in Ultron Unlimited, the duo crafted one of the most intense and brutal Avengers tales in years.
In the story, entire cities are destroyed as Ultron kidnaps Hank Pym and a small group of Avengers in order to use their brain patterns to create a new race of Ultron robots. The stakes are high, the action is big, and the climax is thrilling as the Avengers take on the adamantium-plated android while the world hangs in the balance.