I've seen a lot of misconceptions in this thread, but I'm just going to address this one, then make my own proposition.
I made a Steam guide on how to do "Imitation Empires". Of particular interest for you would be the slavery section:
http://steamcommunity.com/sharedfiles/filedetails/?id=689073779
Litereally everyone used slavery in that age, it was a nessesity of the time. Unless it was an exceptional level of useage, I would just ignore it in the planning.
While the Greek had religion, they were not very serious about it. "Rules to live by - if not too much of a bother." If somebody had proposed to make a Holy War to spread thier religion, that one would have been considered a madman.
So I would rather put them on the Materialist Side. We do remember them best for the various breakthroughs (wich were preserved for us by the Arab world).
It would be better to say that there were many, many kinds of slavery in the antiquity, and among them, some would not be considered as slavery today.
The Greeks were very, very serious about religion. Contrarily to what I've read in an other comment in this thread, Socrates was not condemned because he was corrupting the young, but for blasphemy. There were all kinds of things considered as blasphemy in ancient greece, and in many cases the punition was death.
Now for the materialism - they really not compare with modern day science and technology. Yes there were discoveries, but they were very rarely applied to technology.
Now let's look at each ethics:
Egalitarian : every pop tends to have the same rights. Obviously doesn't work.
Authoritarian : a hierarchical society. Free people on the top, a caste system. Works very well with the greeks : usually you can observe a dominant class made of rich people, the you have the urban citizen, then the country citizen etc until you reach the slaves. Maybe not fanatical authoritarian but still.
Xenophile : as already mentionned in this thread, the greeks didn't really love strangers.
Xenophobe : But they didn't hate them at all. The policemen in Athens were Scythians. Many greeks citizens have been married with greeks from other city states or even barbarians (beginning with Pericles). And it really depended on which greeks (some greek states actually allied with Persia, for instance), and which strangers. Strangers were just not members of the ruling class - but in Stellaris that would make the Greeks authoritarian, not xenophobe. On my opinion, Greeks cannot really be considered as xenophiles or xenophobes in Stellaris, but they would certainly be considered as xenophiles more than as xenophobes.
Militarist : I think that it's a given. Being a citizen meant being a fighter, and ancient greece exalted the values of combat. The Greeks weren't really warmongers in general though, so not fanatical militarists.
Pacifist : there are cases of pacifism among greek city states. But generally it's either situational (they didn't want to make a certain war) or rational (it's useless to fight that specific enemy so let's not). That's not really pacifism.
Materialist : Science, yes. But technology ? Well, there were definitely some technological improvements. But really it doesn't qualify as materialism in Stellaris.
Spiritualist : As I already said, religion was very important in ancient Greece. And not only because blasphemy could get you killed - citizenship and tribes were directly linked to religion. As you grew up, you went through different religious and sacred steps. The Agorai were among the most important places in the city states, and most of the buildings you foudn there were religious. And even when you read the philosophers, you'll see that atheism was not a thing and that most of them were really pious, with only a few exceptions. They would definitely be fanatical spiritualist.
In conclusion, I would say :
- Fanatical Spiritualist + Authoritarian or Militarist
- Spiritualist + Authoritarian + Militarist