Back in 1999 I was happy about the Hypertime concept, as it brought back some kind of parallel earths that have been destroyed while Crisis On Infinite Earths. There were two really amazing story arcs: Dark Flash in Flash 150-159 (1999/2000) and Hyper-Tension in Superboy 60-65 (1999). A lot of Superboy-variants could be seen there.
Superboy, 90s version (Kon-El/Conner Kent)
Different looks from the various timelines, including the Pre-Crisis Superboy alias Kal-El/Clark Kent:
Superboy's opponent in Hyper-Tension was an
evil Superman version called Black Zero.
Some months later on at the Day Of Judgement
crossover a younger version of Black Zero as
Superboy could be seen.