The Great Darkness Saga in LEGION 287-294 was, and remains, the absolute best continuation and conclusion of Kirby's Fourth World series. A thousand years later, Levitz and Giffen provide a satisfying "Final Battle" between Orion and Darkseid, in a Biblical-scale conflict that truly threatens the entire universe.

And I like that it leaves a shroud of mystery over what happened in the 1,000 years in between. That even Darkseid in the long and dormant centuries has forgotten exactly what occurred in the past.

And I warn anyone who hasn't read it, don't waste your time or money on Kirby's 1985 graphic novel HUNGER DOGS, that was his own conclusion to the Fourth World series. This is from me, a guy who loves Kirby. Only in the case it is tossed in a collected Kirby volume would I have this, and only for completeness. But it is possibly the single most disappointing comics story I've ever read. I think I lost I.Q. points reading it.
The other similar disappointment that comes to mind is when a SPIDER-MAN issue (289, if I recall) revealed that Ned Leeds was the Hobgoblin, and with all the clues left to readers for 8 years prior in stories, none of them added up to Ned Leeds. Very cheap and contrived, an insult to loyal readers.