Kart-Hadast is very interesting, as you attested yourself. Quite flexible, and you can take revenge on the Romans for the effective punishment you handed out in that campaign.

Still, I voted Makedon. They have excellent cavalry, a lot of good phalanxes, and start in a difficult position, even moreso on huge population scale. Plus you need to take revenge on the Romans.

The Koinon does have poor cavalry, but great infantry as well. But it may be hard to effectively role-play your generals as the structure of the KH is different from most other nations / factions.

It is a tough choice.