Lex Luthor is the main antagonist of Superman Returns.
he was portrayed by Kevin Spacey who also played David Harken, Hopper, John Doe, Clyde Northcutt & Keyser Soze .
Superman Returns
Lex Luthor tries to take over the world by destroying some of the earth and creating a huge land mass which everyone will have to pay a big amount to live there. He sneaks behind Superman's back to an arctic region that Superman often visits, and he finds out how to access the main computer that will tell him anything he wants to know about Krypton. After that, he finds and steals some Kryptonite so that he use it against Superman, knowing that it makes him weak and deprives him of his powers. Also some of the Kryptonite he stole he put in the substance that helps make land. By doing that he was able to make a land mass that Superman would be powerless on. So Lex was able to get in close and stab Superman in the stomach with a shard of Kryptonite. By doing that, he nearly killed Superman and became rich.
He is last seen stranded on a desert island when his helicopter runs out of fuel. It is unknown if he ever got off.
Trivia
Because of Spacey's Acadamy Award wining performance in singer's film The Usual Suspects (1995), and friendship with the director, Spacey was the only actor considered for Lex Luthor. The writers specifically had Spacey in mind for the part when writing the script. Spacey's version of Luthor has the same comically exaggerated vanity and pompus arrogance of the earlier Gene Hackman version as well as the same strong interest in real estate, but Spacey's version is far less campy and more serious. Spacey later said that director Singer told him to play the character more as "darker and more bitter" compared to Hackman and not to use the earlier portrayal as an inspiration.