Gonna throw my vote at Superman 1 Million as well. If there is a perfect ending to the Superman story, it's 1 Million. Somewhere in the multiverse, there's an earth where Grant Morrison started writing Superman in 2001 and never stopped and never ran out of new ideas. And someday I hope to find that world!
I didn't read Future's End, but from what I know Clark retired and started putting his farming knowledge to work, trying to tackle world hunger. And there's images of him with the beard, slightly long hair, and t-shirt "costume" and while I can't speak for the details or execution of the story, the idea is pretty gods damn interesting and the visual is one of my all time favorites.
I'll also agree with those saying Kingdom Come is an awful Superman. Isn't that the whole point of the story? Clark gave up on humanity because they didn't share his opinion on lethal force, and decades after abandoning us he comes back as a quasi-tyrant forcing his views on everyone, even when that just makes things worse. He's a failed Superman. If I made a list of my favorite Supermen who failed in the biggest ways possible, then KC is at the top of the list (he's better than Injustice at least!) but as far as "proper" Supermen go, KC ranks pretty close to the bottom regardless of age.