I can't believe nobody is giving Macedon any love. Royal Pikemen are really good, they have the light cav to deal with light infantry, and then the Macedonian and Companion Cav to finish the battle, and Macedon is also primed to make money. I like Seleucia, I like Rome. I am a heavy infantry general, so go figure. In my opinion, Militia Hoplites are the best starting unit, at least for defending a town, which as Seleucia you need to do a lot early on. Of course, if dealing with cav archers, that changes your tactics, more light cav and foot archers.

If Rome, I will let the cav archers waste their ammo on my infantry, if I start losing troops I can always go to testudo. Ditto Seleucid Silver Shield Legionaries. Roman armour is good enough that cav archers don't do much, especially after Marius. Now, I rarely face Armenia, they are usually gone by the time I get over there.

Egypt is too good (and with unrealistic units to boot, they would be Greek at this time), though I once saw Pontus take them out and then make it all the way over to Carthage.