"Mayhem over Manhattan"
In which the Masters of Evil are defeated by five Avengers and the Crimson Cowl is revealed to be not an hypnotized Jarvis but actually Ultron-5! I believe this marks Ultron's first appearance in the book. His origin will remain a mystery for some time, and will have far reaching consequences (we will have given up on the Avengers title long before that, by the way). T'Challa still has his half mask on, though this changes before too long.
Roy Thomas is getting into his groove now, and the stories and artwork are tops here. His introduction of Ultron is masterful, and who knew that the robotic menace would become one of the top Avengers foes? I'd put him at #1, though Kang may be tied for that spot as well. Of course, the next three issues have all been rendered moot due to various (and stupid) retcons, but they are great stories still. You just never knew what was going to happen in any Marvel Comic book back then.