Lex Luthor
Superman's most nefarious nemesis. Lex Luthor is a super-genius criminal and his unreasoning hatred of Superman threatens all mankind.
Throughout Superman's rich history, Luthor's hatred has been a constant. In some respects he has been seen as a misunderstood hero, the only human being willing to keep the "alien threat" at bay while at others he is a madman, jealous of the alien being that stole from him the title of "Most Powerful man in Metropolis" (Man of Steel Limited Series circa 1986).
Luthor has been a contant thorn in Superman's side whether it be through indirect means, his control of LexCorp, manipulation of the supervillain community or secret dealings with the likes of Brainiac and Darkseid.
Regardless of his role, as he has had many though the years, Luthor has proven that he can be Superman's greatest enemy or truest friend. It is a complex relationship that, depending on which era you prefer, has shown Lex as Clark's friend, enemy or something in between.