In Final Crisis 7, Superman finally kills Darkseid by singing a song into the newly constructed Miracle Machine. Morrison doesn't let the reader know exactly what song Superman sings, but instead leaves it up to the reader to fill in this particular gap.
Here's how I like to think it went down:
What song do you think killed Darkseid?
Chris Sims presents a good argument that only the unholy sound of Katy Perry could destroy the ultimate evil. If only she would then be encased in inertron to protect the rest of us.
Dorian imagines that Superman would destroy Darkseid with a song about his own existential angst.
Mike Sterling believes that Darkseid could be brought down by the same thing that caused my parents to bang on my bedroom door and scream, "KEEP THAT NOISE DOWN!"
Benjamin Birdie says what everyone already knows: Superman is a big Bikini Kill! fan.
In Kevin Church's world, Superman is singing the Ladyhawke cover.
Dave Campbell helps explain why Darkseid destroyed Indiana first.
Bully also presents a strong case that Michael Parè has something to answer for.
Special thanks to Kevin Church for putting the image and text together and saving me from throwing in a sloppily pasted word balloon and text in ComicSans.