That is a large question.
You have many options in metal
. new slimmer 28mm figures and older hefty 28mm. I prefer the latter since that is what is in my collection.
First off you have factions, the Peloponnesians themsleves, led by the Spartans, and the Athenians and their allies, and the satellite armies of Thebes, smaller city-states (aka poleis if you want to talk fancy). There is the rub.
They're going to be a bit different from place to place.
One of the issues of the wars is that factions began to carry identifying shields to reduce 'blue on blue' casualties-- a situation that occured to the Athenians at one of their battles with Thebes.
Other than that and the Spartan legacy of crimson cloaks you are going to find little uniformity. Over the years in wargaming there has been a tendency to uniformize the Greeks in various stages
certain armor does appear to become less common, but uniformity of helmets and armor never seems to be factor. You will find armored folks and unarmored folks in the same units. tactics changes and unarmored folks are actually embedded to be out runners to help the phalanx stave off nasty skirmishers. Still some of these may wel have been armored as well.
Generally in the Spartan side there is a tendency to lighter gear and the lighter Pilos helmet, but again folks get caried away by uniformity. The Thebans could probably have more of their local favorite Boeotian style helmets. Athenians probably wore whatever was in fashion.
Every side had their haves and have nots armed. So the Spartans had armor for their volunteer helot units, that could have been cast offs, caputured gear, or the older stuff, who knows
a polyglot of armor. Same goes for the Athenians and their metic second line forces.
So choosing and army to start can lead you to the right choices. I like Foundry and Gorgon for Spartans (even though they are a bit too hairly at times). I like figures in the front ranks to be armored, and the back ranks can be lesser armored or only carry shields. My preference is all have helmets, even though that may not be toally 'accurate.'
Some of the Foundry shields are not quite right so I would replace these with better shaped ones. I like the Gorgon shields.
Other brands that work in my collection:
Newline Designs- fit well with Gorgon and Foundry.
Old Glory, these are cheap, they work on their own, I don't mix them with my newer stuff.
Crusader have some nice plain generic hoplites.
Vendel has some very large 28+mm figures if you wish to go jumbo.
Gorgon:
link
Some samples:
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Unarmored Spartan by Newline,did he intend to fight this way or was it because he got bushwhacked in the middle of the night by an Athenian amphibious assault?
picture
Tony Edward's Argives with custom shields:
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A polyglot mix of unarmored mercenaries, Foundry and Newline:
link
JJ