I think it's Cho.  It needs to be someone that would blow you away.  Cho gravitates toward wounded souls (like Summer), is the guy who would be in charge of the team if Lisbon's head got cut off (per Lorelei's request), etc.  I think it's Cho.  Plus the spiritualist in that one episode said he had a "very red" aura, which I take as a winking clue, not a "real" clue.  But I still think it is him.