It depends on what you mean by win. If you go by who has more impressive feats, it's Luthor, hands down. At one point, he became omnipotent and omniscient. The only reason he became human again was basically because of his hatred for Superman.

If you're familiar with the DC comics system if classifying intelligence, Lex is "overqualified" as a twelfth level intellect, which puts him on par with Braniac, if I remember correctly. If you're not familiar with it, if you combined the minds of every 20th century human into one huge brain of sorts, it would only be a 6th level intellect.

(More to come, then I'll do another post with what I can find about Stark)