In my opinion, the Skiritai Commandos of the Spartans are the 'best' units, for the following reasons:
They do not cost ridiculous amounts of resources like elephants so that you are always able to afford quite a few of them.
They are significantly stronger in attack than other citizen-infantry and have more health than them. On top of that, they are able to work for your economic boom between battles, unlike the mercenary units.
They walk as fast as archers and are more durable than pikemen.
If you have a large number (more than 40) of Skiritai Commandos, you can use them as siege weapons. They are quite resistant to arrows from fortresses or towers.
They do not need special buildings or expensive technology to unlock and train. You can mass produce them from barracks.
In a unit vs unit battle, they will certainly lose to champion units and hero units. However, considering the cost-effectiveness, they are excellent.