Just things with Superman in them, I'd recommend JLA/Avengers.
Superman is especially prominant, along with Cap. In general it's a must see event for being a genunely epic feeling event, that managed to be fun. You can almost call it a cap on comics before the New 52 broke DC..
All-Star Superman [Morrison]
Superman Beyond [+Final Crisis] [Morrison]
Superman: Secret Identity [Kurt Busiek]
Superman: Red Son [Mark Millar]
Up, Up and Away [Kurt Busiek & Geoff Johns]
Superman: Last Son, Legion of Super-Heroes, Brainiac by Geoff Johns
Superman: Birthright [Mark Waid]
SUperman: For the Man who has Everything [Alan Moore]
Superman: Whatever Happened to the Man of Tomorrow? [Alan Moore]
Action Comics 1-16 [New 52] by Grant Morrison
Superman: For All Seasons [Jeph Loeb]