I can't think of anyone who can really beat Thrawn. Someone out there could give him a challenge, but he could outwit anyone. He can make something out of nothing. You would have to pit him against a Galactic Government in order to have a good match-up, and we saw what happened when he went up against the New Republic. If it weren't for the Noghri, Thrawn would have swept the New Republic aside.
Pelleon could maybe take on Thrawn successfully, and that's because Pelleon was Thrawn's protege.
Garm Bel Iblis was the only guy that I recall that actually could go toe to toe against Thrawn, but then the Corellian was fairly unorthodox.