Titano is criminally underused.
Actually, most of Clark's rogues are underused, outside of the big ones everybody keeps going back to, like Lex, Brainiac, and Zod. I think that's why people say the rogues gallery is thin, because there are some incredible villains in that roster.....they just rarely get used, and when they do pop up, it's usually for little more than a brief cameo. I mean, when was the last time we saw Maxima show up to bother Clark? And Imperiex was literally just one storyline (a big, long storyline, but still just one story).
Of course, I'd like to see Maxima handled as more of a....not an anti-hero, but a unreliable ally. Maxima actually has some pretty noble goals; she's just trying to do what is best for her empire, and her culture makes it hard for her to understand or respect Clark saying "no." Great stuff to work with there. Me, I'd bring her back by having her marry Vartox and invite Superman (and family!) to the wedding.
Can we add Lobo to the list? He's more of a solo star than a rogues gallery member, and he didn't get his start in Superman comics, but I feel like he deserves a spot on the list.