Avengers Forever #1-12
Kang and Immortus are at war, with Immortus trying to stop humanity from threatening the galaxy and Kang figuring that this priority of his future self offends his honor. A team of Avengers are gathered from different time periods with Rick Jones, Wasp and Antman from the present, a despondent Captain America, a crazed Yellowjacket (from that time Hank Pym had a breakdown and believed that he killed Hank Pym) and cocky size-changing Hawkeye from the past and Songbird and Captain Marvel (Genis-Vel) from the future.
After an initial showdown in limbo, they’re sent on missions to the past and future as they uncover the secrets of Kang and his many variants. One group encounters the Avengers of the 1950s when their interest in a Skrull impersonator of Richard Nixon is misunderstood. Another group goes to the west, which makes former carny Hawkeye giddy. The third group goes to the future of Killraven when Black Panther and a pregnant Jocasta lead what is left of the Avengers.
There are a lot of major retcons, but in a story about time travel it feels natural. Examples include the exact relationship between Kang and Immortus, as well as the exact relationship between Vision and the original Human Torch. It might seem indulgent, especially with big moments that don’t involve the Avengers in this story, but it’s fine for a story that is so obviously a love letter to the whole franchise. It is the type of story that makes the other comics it references better. And they do their homework, so something that might contradict early Avengers issues turns out to be supported elsewhere (a twist with whether there can be more than Space Phantom connects to a What If? Issue, the Avengers forgetting a Silver Age story due to time travel shenanigans figures into the plot.)
Years ago, this story was in first place on a list of 40 best Avengers stories meant to commemorate the 40th anniversary of the Avengers. I don’t know if I’d go that far (my personal favorite Avengers story was published at around the same time- I’ll cover it in about a week), but it’s fun and epic and says something about the characters and the purpose of humanity.
It seems to have an impact on the depiction of time travel and Kang in the Marvel Cinematic Universe, although part of this may be how expertly Busiek and co-plotter Roger Stern hang together all sorts of different elements of Marvel Comics.