The Original Death of Superman
In the early 1960s, Edmond Hamilton and Curt Swan penciled the story of Superman's dying days as an epic Superman novel in three chapters. This is that story.
When Superman comes into contact with a space born chest containing samples of 'Virus X' a disease lethal to Kryptonians, he finds himself weakening and dying. At first he tries to complete a number of projects, but when he's too weak to continue, he calls on Supergirl, and a number of his colleagues, to take up the burdens.
If there's one thing an issue like this does, it really summarizes all the detritus that have piled up in the Superman comics in the first quarter century such as the inconsistencies and hazy definitions of Superman & Supergirl's powers (Superman's power of 'total memory'; Supergirl pushes an entire planet at one point in the story, but then she needs the help of a bunch of Kandorians just to shape a mountain in Antarctica; etc.).
Also, Lana and Lois seem more dim-witted than ever - how could they not know Clark was Superman given the events of this issue? And one begins to wonder about Superman's priorities - if this stuff was all so important to the survival of Earth, why did he put it off until he knew he was dying?
The science in this issue is really screwy at every turn - particularly with respect to what these super beings do to the environment. But the workings of chemistry, physics, etc. just aren't much respected here either.
Ultimately, Superman's final farewell is poignant, but all its impact is lost in pages of super-beings doing a bunch of random heroic deeds while Superman sits on the sidelines in a lead glass box.
Perhaps when I get around to reading the Roger Stern 'Death of Superman' storyline decades hence, it will be better?