@entropy_aegis: Lex's intellect = Superman's strength. The stronger the Superman, the smarter the Lex. And Superman is pretty damn strong.
Also, beings like Brainiac often refer to him as the smartest mind on the planet. Superman often calls him the smartest man on the planet and wishes he could use his intellect to do good instead of evil--he's never told Batman that and Superman is often used to prop up Batman--and, like Michael's self proclamation, Lex often calls himself the smartest man alive.
Batman isn't top 3, I though Oracle was smarter than him? How would that work if Luthor is number 1, and Michael is number 3? And Michael is number 3, DC had a chance to retcon him classifying himself as the number 3 with the N52, but he kept it .