I voted for ancient greek. IMO there's just much more freedom to create new challenges in that era. Of course you could introduce magic and mythic monsters into a classical setting, but it would lose uniqueness this way. Using the ancient setting you have all avenues of mythical events open, literally anything can happen. But this doesn't mean the human-human part has to come up short.
Ok, so the Troian War has been fought out, but there can always be some other conflict somewhere and wars CALL for heroes, the fighting (Achilleus: Fighter 23) and the less combat-oriented ones (Ulysses: Fighter/Rogue 10/9, Nestor: Fighter/Rogue/Aristocrat 13/2/1 but venerable age). Continuing the example we can say, that while Achilleus was the singlemost powerful character, it took much more than just his martial ability to win the war - in the end it was Ulysses sneakyness and the interventions of the gods (spells cast by clerics in a DnD conversion), that decided the war. And without characters with social abilities convincing the right people, commanders making decisions of tactic and strategy, they wouldn't have won either.
Imagine the possibilities: The party is the elite force hidden within the Troian Horse and has to break the resistance from within - challenges for all character classes. The rogues can sneak about, assassinating, opening the doors, etc. The cleric's invocations hide the party from the view of the opposing gods and the fighters can unleash hellish slaughter in the streets once they're found out.
This is of course only an example, but it shows the possibilities of the ancient era. Mix the Troian War with Ulysses adventures and a good deal of mythological creatures and a divine/titanic subplot and you have an excellent setting.
If you do this, I'd recommend put great emphasis on the intervention of the gods. In 'standard' DnD they have quite an influence, but in an ancient greek setting they (or the titans) would behind every major development (which doesn't take away from the human's ability to make differences and decisions with or against the will of the gods, although they have to face the consequences of course).
Magic items would either be created my legendary, half-divine mortals or directly by the gods or maybe titans on the other side. This could lead to interesting situations as these blessings could become stronger or be taken away depending on the wielder actions or simply not work in specific situations - A sword blessed by Aphrodite could refuse to attack a creature of the wielder sexually preferred gender or Ares could temporarily withdraw his blessings from a weapon used to attack a follower of an allied god.
Clerics would directly call to the gods and ask for blessings and miracles, which would of course have to be related to the gods domains and the god decide to grant or deny them on a case by case basis - another of these blasted titan-spawn! Here, I grant you the use of my own sword for a while. - asking for favors again? I'm not in the mood ... maybe, if you perform some more sacrifices. Faith would be much less required than obedience and sacrifices.
I hope, some of my ideas help you
