Again, some good choices here (Death and Return of Superman, in particular, is underrated.) I’d nominate:
All-Star Superman: Grant Morrison’s 12 issue love-letter to the character.
Superman For All Seasons: Jeph Loeb and Tim Sale, doing for Superman what they did for Batman in The Long Halloween.
JLA by Grant Morrison and Howard Porter: though not a Superman series per se, this still has some all-time great Superman moments.