Mouserocks, I love Kingdom Come, too, but I hesitated to recommend it because it might be a bit harder to get through if the reader isn't familiar with some of the other characters. It does focus on Superman, though, and it's a great read.
Another Elseworld story that's really good is Superman: Red Son. It basically asks, "What if Kal-El landed in Russia instead of Kansas?"
True Brit, which I mentioned above, is, "What if he landed in England?" Actually, a lot of my favorite Elseworlds stories are, "What if he landed somewhere else or were picked up by someone else?"
Speeding Bullets = What if the Waynes found him?
The Nail = What if the Kents got a flat tire that day and didn't find him? This one is an excellent story, but Superman is barely in it, and it doesn't make nearly as much sense if you're not familiar with a large number of DC characters. The Kents are in it, though, and they're awesome. They end up doing just what you'd expect in the situation from people who would be willing to suddenly take in an alien baby.
