Superman trades I'd recommend:
The DC Universe Stories of Alan Moore (for the stories For the Man Who Has Everything and Whatever Happened to the Man of Tomorrow?)
Superman: Secret Identity (in this trade, written by Kurt Busiek with art by Stuart Immonen, we are shown exactly what it is that makes Superman an enduring icon, a hero among superheroes)
The Death of Superman/World Without a Superman/The Return of Superman (these 3 trades comprise one of the most harrowing times in Superman comics, and while certainly not the greatest Superman story ever told, a strong one in all regards)
Superman: For Tomorrow volume 1
Superman: For All Seasons
JLA volume 2 (American Dreams) and volume 4 (Strength in Numbers)
Single Issues:
All of Superman: Confidential
Action Comics #775
all of All-Star Superman