Who's This? And now for the second half of the Phantom Zone entry in Who's Who vol.XVIII.
His first story was told in Reign of the Supermen, a tale from Adventure Comics #289 (1961) in which he posed as a super-powered Pa Kent. He would return in more modern Superman tales as one of Zod's cronies, appearing on the Superman animated series and in the Man of Steel movie as well.
Another character discussed in Reign of the Supermen, with Zod, arguably the best known inhabitant of the Phantom Zone (at least, in comics fan circles). He's been a high-profile member of the Legion of Super-Heroes since the early 60s, the only character on the list to get his own entry in Who's Who, and even took over Action Comics for a while in the more recent past.
Who else? I was probably going to go western, but then I had any idea for a very different genre character.