It seems like all my favorite Superman stories are out of continuity, All-Star Superman, Kingdom Come, Whatever Happened to the Man of Tomorrow?, Red Son. These stories are able to get at the defining essence of Superman by pushing him to extremes that do not mesh well with a monthly continuity intended to be never-ending.
Within continuity I find Superman a hard character to care about. His powers are accidents of birth, not earned, and so vast that he is never in any real jeopardy. The only truly remarkable thing about him is that he chooses to practice self-control and adheres to a moral code, but his impeccable boy scout nature also makes him a dull character because there is no possibility of actual character development. Superman is better used as a Deus ex Machina in other non-Superman comics where he can come swooping down at the last moment and save the day.
Morrison solves this problem by treating Superman like a mythological figure, like a sky god. He mixes epic level threats with the emotional impact of Superman facing his own mortality. It is not easy to make Superman genuinely interesting, but Morrison pulls it off.