Generally you should go for the richer enemies first when playing as the Seleucids. That means Egypt, Pontus, then Parthia/Armenia in that order. It's just a lucky coincidence that this is the same order of threat they pose to you.

Once the Nile and Turkey are yours, it doesn't matter how teched up Parthia and Armenia are, your wealth will always beat them. From there, it's onto Greece, then Rome!