There was no one Greek polis that was the main one to take on the Persians in these wars. Instead, many of the city-states were involved in the war. Perhaps the two most important, simply because they were the largest and most powerful, were Athens and Sparta.
Both the Athenians and the Spartans played major roles in some of the most important battles of the wars. For example, it was the Athenians who fought at the Battle of Marathon and it was the Spartans who provided the men who fought the famous Battle of Thermopylae.
The Athenians and Spartans did not fight alone, but they were the biggest and most powerful of the poleis and so they could be called the main participants in the war.