I think Dark Knight Returns is great, but I always saw Kingdom Come as the superior "DC heroes several decades in the future" story. DKR pretty much gives Batman all the praise while not really giving Superman that good a portrayal (and the sequel just gave the middle finger to him and Wonder Woman). Kingdom Come showed all three having gained flaws as the years went on that feel in-character for them, and coming together in an awesome showdown. Superman vs. Batman is never usually exciting for me because the fight always is one-sided depending on when the kryptonite is deployed (and thus making every fight play out the same). Superman vs. Shazam, though? That's some good shit.