The One Who Knows Everything
So I was looking at the old chapter titles, and this one stuck out.... 397: The One Who Knows the Truth.
Of course, this was the chapter in which Obito/Tobi told Sasuke the truth about Itachi. However, during that time, he was still under the guise of "Uchiha Madara."
Perhaps "the one who knows the truth" is "the one who knows everything." By that assumption it would be either Obito/Tobi, because he shared the knowledge with Sasuke, or Madara, because that was Obito/Tobi's supposed identity at the time.
My theory is that Sasuke has already met Obito/Tobi and knows about Madara and understands that they knows things he doesn't...as learned from his time in the cave with Obito/Tobi. He also later found out that Obito/Tobi was leading the Akatsuki when Obito/Tobi recruited him to catch the hachibi. So he might be going to either one of them (most likely Obito/Tobi, who he has already met.)