Joker is wonderful, but Lex is one of the best literary characters ever made, imo, especially as a foil to Superman.
On one hand, you have Superman, who is obviously not human, but embodies all of the greatest traits humanity has to offer. On the other hand, you have Lex Luthor, who is a human and was given every opportunity to succeed, and he embodies all of the worst traits humanity has to offer. Every awful thing, every crime, every shitty thing humanity can think of - he's done it. And beyond his actions, he's also a coward, an opportunist, a sleaze, insecure, addicted, self-serving and every other negative adjective out there.
And the great tragedy of the character is that he doesn't actually need to be that way to get what he wants. He wants to save humanity, and he's got the brains, wealth, connections, and position to do it. There is absolutely nothing stopping him from getting what he wants... except himself. He can't let go of his anger and jealousy and hatred, and it completely blocks him from his own goals. He could be everything he wants to be, and a greater hero than even Superman, but he just can't shake his own insecurities. And those insecurities and hatred cause him to be the worst human being on the planet instead of Earth's savior.
It's tragedy on a Shakespearean scale, wrapped up in a comic book.
