1960s: “The Death of Superman”
Superman vol 1 #149 by Jerry Siegel and Curt Swan
The 1960s was a good decade for Lex Luthor, as you might be able to determine by how full to bursting the honorable mention section for this decade is: he gains a first name, an origin that connects him to Superboy, a family of his own in the form of his sister Lena Thorul (and in 1970, a niece, Nasthalthia), a partner in crime in the form of Brainiac, and a whole planet (with a red sun, naturally) where he can plot against Superman and even finds a wife with whom he has a child.
The selection here, “The Death of Superman,” is an imaginary story (i.e., a non-canonical “what if” type thought experiment), but is likely not only the best example of that sub-genre of story, but probably the best Luthor story of them all. This book-length three-part novel shows the depth of Luthor's enmity for Superman: he hates him so much he would cure cancer just to kill him.
Best of the rest: “The Showdown Between Luthor and Superman!” (Superman vol 1 #164), “Luthor—Super-Hero!” (Superman vol 1 #168), “The Death of Luthor” (Action Comics vol 1 #318-319), “How Luthor Met Superboy!” (Adventure Comics vol 1 #271), “The Army of Living Kryptonite Men!” (Superboy vol 1 #86), “The Face Behind the Lead Mask” (Adventure Comics vol 1 #300), “The Conquest of Superman!” (Action Comics vol 1 #277), “If Luthor Were Superman's Father” (Superman vol 1 #170), “The Triumph of Luthor and Brainiac” (Superman vol 1 #174), “The Team of Luthor and Brainiac” (Superman vol 1 #167), “Lex Luthor Meets the Legion of Super-Heroes” (Adventure Comics vol 1 #325), “The Musical Murder of Superman” (Superman's Girl Friend Lois Lane #64-65)