I'm not so sure there are any really weak factions in the game. I don't like Numidia and most of the Carthage infantry is poor, but I think the game (well, without the silly Egyptians) is fairly balanced.

But...to beat the Seleucids, I do think Parthia would be decent, and the Dacians, Germans or Gauls would not be too bad either because of the arrow storm you can get. I just don't have much experience killing the Seleucids, just who I don't like to fight when I use them.

Azi