Alright, it's not Ketchum or me, because we're the only ones alive and if one of us were the Thing we'd be at parity and have already lost.
It's not Slapsticks or Lefanis, because they died after the second Thing appeared.
It's not Telrunya because he was the original Thing.
It's not Barek because we saw his corpse, and Fury said the Thing was living in its new host.
That leaves Penchant, Zakilevo, Lanyon, Sonya or D'Espana.