Anyone you think could challenge him? I'd kinda like to have him in my Kaggath Series.
In a battle of wits. Very, very few. Maybe Admiral Ackbar. There was this one Galactic Alliance Admiral (Bwatu or something) that beat the Thrawn simulator, but I don't really think a simulator can replace the real thing.
If anything, you would have to have someone with Battle Meditation in order to have a good chance at winning. Maybe Yoda, or Sidious. Possible Bastila, but she's not really a military commander.