Schumann Toccata--I know it's not an "etude" technically, at least by title...
Well, I've played the Prokofiev Toccata, but not the Schumann. I can tell you that, while the Prokofiev is tough, it isn't even close to Rach's op. 23 #9 or the Coloana fara Sfarsit.
Godowsky's transcription of the thirds etude