Someone once explained me that he thinked Liszt might have meant that the etudes cannot be played slowed down(or with crappy technique). The musical effect is only created when you have the transcendental skill to execute them.
So the magical/supernatural effect is only created when they are played by someone with the proper technique.
But if this is what Liszt really meant, no idea.
I don't think he meant a plain 'best'. But that might be wishful thinking, arrogance is not a virtue.