If you're talking financial corruption, it's probably a tie between Grant and Harding.
Your friend is probably not willing, or perhaps capable, of defining his terms.
In the modern, but pre WW II era, it would be Harding. Post WW II, without a doubt the attempt at creation of the Imperial Presidency by the Nixon gang will go down in history as the most sinister and corrupt. With the sole exception of Doctor Kissinger and the opening of relations with China, Nixon runs away with that inestimable title. The internal conduct of his gang of thieves, who were unrepentent when exposed, was an all-time, Olympic quality disgraceful challenge to our democracy--and, that cretin on talk radio, Liddy, remains unapologetic for his criminal conduct. Of course he's sorry he got caught. A truly contemptable character.
Harding was going to be my answer as well. The Teapot Dome scandal was huge at the time. Though I think Harding had less to do with it than Wilson did. Harding had the misfortune of being in office when Fall leased the land to Mammoth.
Corrupt. Here are some interesting synonyms: Corrupt people are those believed to be evil, depraved, perverted, debauched, defiled, tainted, stained. They are the type that undermine, impair, damage, abuse, mistreat, dishonor, violate, disgrace, violate, and cause the nation to degenerate. Golly! These all fit Clinton to a T. But so do many of the terms fit LBJ. A few fit Nixon too. Carter gets several after his name. So does JFK, not to mention FDR. Among RATs, it's hard to chose anyone worse.