Really, there's no way that it is not Reger's Bach variations! Le Festin and Godowsky's passacaglia are very, very difficult, true; but consider the variety of techniques required and the LENGTH (which implies the stamina needed). And don't forget about the 8 pages long fugue at the end! I've once read the Hamelin said this is one of the most difficult things he ever played and it seems very plausible for him to say that.