#18. The Avengers vs. Ultron
Ever since he was accidentally created by the Avenger Hank Pym, the evil artificial intelligence, Ultron, has been a thorn in the Avengers' sides. Over the year, the robot has continually updated himself, becoming more powerful and dangerous each tim. But possibly his most dramatic battle with the Avengers came with the nineteenth version of Ultron, in the pages of Kurt Busiek and George Perez's "Avengers" run – the storyline that ran through "Avengers" #19-22 and was titled "Ultron Unlimited."
The concept of the storyline is that Ultron has decided that he does not want to simply wipe out humans from Earth – he wants to repopulate the world with his own people: robots. He begins this attempt in horrific fashion as he enters the small European country of Slovenia and proceeds to slaughter the entire human population in three hours. He sends a message to the horrified public watching at home – do not come into this county or suffer the same fate.
Meanwhile, he has also kidnapped the Avengers that he considers "family" and intends to use their brainwaves to base his new world population of robots on, much like the way he earlier based his intended robot bride Jocasta on Wasp's brainwaves, the android Vision on the brainwaves of Wonder Man and the robot Alkhema on the brainwaves of Mockingbird.
It is during this story that we learn for the first time something that probably should have been evident to readers earlier (it's somewhat surprising it took decades until Busiek came up the concept), which is that Ultron's mind was based on the brainwaves of his creator, Hank Pym, who happens to be among the Avengers kidnapped by Ultron.
The Avengers ultimately decide to invade Slovenia, resulting in many interesting battles within the country as the small band of heroes seem to be overmatched by Ultron's apparently unending supply of robot drones (hence the "Unlimited" part of the story's title). During the course of this war, the Avengers have to face off against all the earlier Ultrons, each of whom was enough to fight them to a standstill in previous years.
Ultron is quite confident that his minions are more than enough to defeat the Avengers. That same confidence leads to one of the coolest dramatic entrances ever (and winner of a Wizard Award that year for Best Moment) when the Avengers burst into Ultron's lair, looking quite ragged, with Thor speaking for the entire team when he declares "Ultron, we would have words with thee!"
This turns the tide, and ultimately, Hank Pym is able to redeem himself and save the day.