So, I think most people would immediately say Macedon, Egypt, or Seleucid.
But I think actually the best would be Bactria.
Their roster has all the essentials of a top tier Hellenic roster:
* Cataphracts
* Elephants
* Superior shock cavalry
* Thorax Sword and Pikemen
* Horse archers, particularly the mercenary Scythians
* Light melee cavalry
* Heavy melee cavalry
* All the best foot archers
* They also get a superb high end hoplite unit which the others don’t. It’s ideal to bracket the flanks of pike lines.
Their only weakness is the lack of a cheap shock cavalry unit.