If you think about Superman as a story with thematic components in which the characters have been carefully designed to connect in narrative harmony, all the suggestions for Lex Luthor on this board are extremely misguided. Rather, a physically weak, conniving, squirrelly-looking Lex Luthor is essential (like in
Birthright).
The reason is because that physical look is the perfect contrast to Superman. Superman's power is in his
physical gifts (physical strength, speed, flight, etc.) and Luthor's power is in his
mental gifts, a genius Einstein-level intellect that he uses for evil. The key is that
their power is equal. Despite Superman's astonishing physical power, Lex is able to compete with him through
sheer intelligence, while his physicality is weak and unimposing, thus
visually emphasizing his great intelligence as the source of his power. Take this panel from
Superman: Birthright:
Lex is
small, skinny, has
harsh, pointy features, almost like an insect, the oppostite of Superman, conveying his physical weakness, but also a ferocious intellect, his contrasting quality to Superman.
To take away that element of the character and turn him into a mere overconfident thug (and cast so many of the names being thrown around in this thread) is to take away the fascinating dynamic between hero and villain, the feeling that, although they are morally opposed, they are two halves of a whole, two extraordinary creatures who are capable of achieving anything due to their gifts, but they use those gifts differently because of the values that they've adopted and moral choices that they've made. That characterization of Luthor adds such tragedy to his character because, for instance, if he had made different choices and decided to use his intellectual power for altruistic goals the way that Superman has decided to use his
physical power, think of all the amazing achievements he could have bestowed on humanity (achievements that, most likely, no one but him is capable of).
This question of how to use your natural gifts is central to any Superman story. The purpose of Superman is to use the main character to inspire us to use whatever natural gifts
we may possess for the good of humankind, to find the "Superman"
within ourselves, if you will. Since Luthor is the primary narrative foil to this message, he must also possess a natural gift that he abuses, displaying the alternative and
immoral choice. Having a lot of money and being a bully (the way that he was portrayed in
Superman: The Animated Series, for instance) is not a natural gift and is not thematically consistent or meaningful. And casting an actor who is physically strong, who is tall, muscular, handsome, etc...
ruins this dynamic, diluting it to the point that it vanishes completely from the story, thusly creating a far less meaningful story.
This is, of course, a mistake because while it would be easy to just say, "Oh, this guy would be cool as Lex!" without giving much thought to it, these characters have such potential to create an entertaining as well as truly touching and powerful story and I believe that the way the characters were initially conceived (Luthor as the genius scientist) was rich and ripe for inspirational storytelling. Superman was conceived as a classic myth and to change Luthor into a large, tough corporate thug and take away the dynamic between hero and villain betrays the roots of Superman. In classic myth, the hero and villain are opposites in every way. Superman is tall, muscular, physically strong. Luthor is small, skinny, physically weak, but intellectually powerful. To do anything else would be to simplify the story and diminish the potential for meaning and drama.
An actor who resembles the above drawing from
Birthright should be cast. An actor like Jackie Earle Haley who Snyder worked with in
Watchmen. Another actor that fits the mold is Mackenzie Crook, from the
Pirates of the Caribbean movies. Or even message board-favorite Crispin Glover. I'm not saying any of these actors should be cast, I'm saying that this is the
physical type that should be looked at because it creates the movie that's richest with meaning and drama and classic myth-like power, something that audiences undeniably respond to.
Imagine if you had a handsome, charismatic Superman going up against a villain that looked like this (it would be visually rich and striking and the meaning of the story, the "brain vs. brawn" dynamic would be right in the audience's face and so much fun. Perfect.):