My copy of Davidson's Courtesans and Fishcakes: The Consuming Passions of Classical Athens is packed away, so I'm working from memory here, apologies for any memory glitches, but here's what I have off-hand.

There's a running joke in New Comedy and in political speeches from the time that it's easy to spot a spy in Attica, or to spot an Athenian spy elsewhere. Athenians are the only ones who can stand the cold unsweetened, unsalted barley soup that they ate two meals a day, breakfast and lunch. The classical period was a dry period, and Attica didn't have all that much horizontal farmable land to start with. Athenian farmers raised sheep, but didn't slaughter them for meat all that much; mostly they were raised for wool, one of the country's main exports. I think they also raised chickens, so dinners would have at least occasionally included chicken.

On the other hand, by the classical period, the Athenian calendar is an average of one neighborhood or city-wide feast day every three days. Rich people paid "taxes" in, among other things, meat -- mostly sheep and goats until Athens got temporarily rich, then imported cattle; the prime cuts would be roasted; once the priests and the donors were fed, the remainder of that would be sold or distributed first-come, first serve; the non-prime cuts would be boiled and served first-come, first serve to anybody who showed up.

Note that this part of the diet is 100% consistent, in Athens, for slaves, landless peasant workers, merchants, and even the wealthy; one thing that Athenians were mocked for was that slaves in Athens ate as well and dressed as well as their owners. The big class signifier, especially late in the Classical period, is how much fish they can afford to supplement their diet with. The Athenians very badly over-fished the nearby waters, sailing farther and farther out to bring back less and less fish which sold at ever higher prices; the mark of a rich man, especially once the capital controls and progressive income tax system started to break down, was that he could afford to serve a lot of fish at his parties, and the mark of an untrustworthy man was that, at a party, he would rush towards and quickly eat up all the fish, before anybody else could have some. "Fish-glutton" (opsophagos) was a standard political insult that meant "has no control over his appetites," a serious insult in Athenian culture, on par with "sex addict" (katapugon).

Right before it all came apart, the Athenian government got a huge cash influx from two sources: the largest silver deposit ever found (which they mined using prison labor) and renting out the Athenian marines as mercenaries. They used that money to import huge amounts of food from Egypt, mostly grain for bread, to hand out to the injured veterans of Athens' then-endless wars and to the orphans of those who died in those wars.

The ancient Greek diet is something I've posted about in the subreddit before, so I'm partially using my own answers- any resemblance to the answers of one /u/Daeres is entirely intentional!

The first thing is that no ancient Greek would have eaten breakfast, lunch, or supper, not as we'd currently understand them. The three meal structure that so many societies now follow (and yet so many societies do not) is a relatively recent development and is partially a result of industrialisation. Generally speaking the Classical Greek day would involve four meals, but possibly as many as five. The major four meals were a light morning meal (akratismos), a light midday meal (ariston), a light afternoon snack (hesperisma) and the main evening meal (deipnon). For those who partook of them, a symposion would come after the main evening meal, but I'll discuss those more below.

The second is that anything represented here isn't just variable by social status but also location; there were many local identities within Greece, not just of various city states but also sub-ethnicities within the wider Greek world. What is true in Athens is not necessarily true in Syracuse, or Thebes, or Ephesus. And unfortunately we rely on upper-crust Athenians for a lot of information regarding Classical Greece as a whole, so there are some elements of these descriptions that may potentially be more localised practises in Attica (rather than Greek-wide practises), but if that's the case we simply lack the information to distinguish.

The third is that social status in these ancient Greek societies rarely conforms to our modern class structure. It's not that relative economic wealth had no social bearing, but many Greek cultures had social status being inherited rather than necessarily being a mutable thing, and it was also frequently tied to land-ownership. In Athens it had been common to attempt to divide social class by the amount of wheat your land produced, for example.

Even if you were relatively low down on the social ladder, a free Greek could expect to eat at least three of those meals a day. In terms of general foodstuffs, even the poor would be expected to have access to a number of basic amenities; wheat and barley (both for flour and porridges/gruels), olives, figs, lentils, onions, chickpeas, broad beans, pork sausages, grapes, fish/seafood, cheese (mostly from goats and ewes, and resembling feta cheese in texture) and eggs. In addition, though it might be poor quality all expected access to wine. This might be closer to vinegar than wine at times, however. Milk was stereotyped as being drunk by farmers and shepherds, 'rustic' folk as t'were.

In addition to this, citizens of cities would also get meat via animal sacrifice. The usual Greek practice was to kill the animal, then inspect the organs for omens, put the major bones onto a fire as the god's portion of the meal, butcher the animal, cook the meats and internal organs, and then to divvy them out to everyone partaking in the sacrifice. Part of having citizenship of a city was having the right to partake in sacrifices (though attendance was usually compulsory). The cities that were oligarchies would likely have had less % of people partaking in this kind of meat. The most common animals you find in this scenario are sheep, cattle, goats, and pigs. Occasionally you hear about more exotic sacrifices but those are almost always for the rich in their own cults.

Once you start moving into higher incomes, then you find that meat is more plausibly acquired outside of a) preserved meats like sausages or b) sacrificial meat. Meat was always associated with sacrifice by the Greeks, but they never assumed it was the only way to go about acquiring it. In addition a wider range of fruit starts to become available on a regular basis; what fruit this might include varies a lot depending on the period, because over time the Greek world began encountering more and more far-ranging fruits. For example, apples were a proverbially exotic fruit to Archaic Era Greeks, frequently appearing as these mystical items. This continues in later periods, but the apple becomes somewhat demystified once it is brought over from Anatolia sometime around Alexander's lifetime. Pomegranates were more common but still a little exotic. Also around Alexander's lifetime we start seeing more references to apricots and cherries, which had previously been almost unknown to mainland Greek persons. Almonds, which had been seemingly rare in Mycenaean times, were very common in Classical Greece. Walnuts were highly prized, variously being considered medicinal and an aphrodisiac. Honey was valued both as a sweetener and medicinally in its own right; it was never fully out of the reach of Greeks, but beekeeping was a fairly elaborate activity and it does seem to have been more common in higher class diets. And we have the elephant of the room of spices, or at least those not common/grown in Greece itself- those which had to be important from thousands of miles away such as pepper and cinnamon.

However, in all honesty the major difference between the food of rich Greeks and relatively poor Greeks does not seem to have been principally in the kinds of foods eaten. Rather the difference between them was in the regularity of certain elements (such as meat and certain kinds of fruit), the quality of elements (such as high quality wine), the care in preparation (very soft white breads vs those likely to degrade your teeth over time, the presence of spices to flavour as well as to preserve, elaborate dishes as a whole) but most especially the quantity of the spread in general. A symposion was effectively an additional meal during the day, and was for all intents and purposes a banquet. This was an activity principally engaged in by individuals we might term aristocratic, and was very much celebrated in 'elite' Greek culture. It's not that feasts were unknown to ordinary Greeks, but they often accompanied festivals and sacrifices (and indeed the sacrifices often provided the actual feast!).

Now, in the later Hellenistic era, some of this changes. A number of the fruits that had been relatively rare became more common as Greek-speaking kingdoms spread across Asia in Alexander the Great's wake. The Seleucids seem to have successfully been able to cultivate rice in Syria and Mesopotamia (unless some unknown state introduced these earlier) which doubtless would have entered into the diet of some of the Greeks in the Seleucid Empire. In general, when we begin to talk about Greeks living as far away as modern Afghanistan we begin to loose our ability to talk for certain about a relatively unified Greek diet. In addition, we also have to assume that in many of the Archaic/Classical Greek colonies far outside Greece itself their diet may have varied from the Greek norm as well.

So far as I can tell, many of the Greek ingredients of modern cuisine were present to one extend or another in ancient Greece but they were not used in the same way when creating dishes. Flatbreads were fairly common, but these would often have been made from barley and not wheat, and I can't say if they'd resemble a modern pita. Also, having myself been around Greece I know that local cuisine tends to vary anyway. As for modern elements of Greek cuisine which would have been absent- tomatoes, zucchini/courgette, eggplant/aubergine, potatoes. In addition, many of the most famous Greek dishes do not originate in Greece itself and are instead a result of the Ottomans- moussaka, to take a very prominent example. But fear not; the British did not invent fish and chips either, not Scots the haggis.

Can I piggyback on OP's cool comment to ask a related question? Too late, I am already doing it: Were ancient Greeks similarly long-lived, in comparison with contemporaries who ate a significantly different diet? Because one always hears that Greek people are long-lived thanks to, I don't know, olive oil.