i have a Superman/Batman collection from 1992 to 2002. Death of Superman and Knightfall brought me into love of comics. My collections ended with Batman: Hush and Superman: For Tomorrow. After that i was done, both stories felt like good ways to pull out at the time.
Now with the big reboot in September, I won't deny that i'm interested in jumping back into Superman and Batman.
from that 1992-02 collection, when it comes to superman and batman, stories I would recommend to read is: Knight(Fall, Quest, Ends) Doomsday, Funeral For a Friend & Reign of the Supermen. Batman: No Man's Land, Superman: Our Worlds at War, Superman:Reign of Emperor Joker, Batman: Hush.
And I cant recommend DC's Kingdom Come enough.