Are you including slaves in this? I'm going to ignore slaves.

So... ancient Athens? I won't argue that their lives were particularly grand, but as you say, relative to timeframe, they're pretty decent.

Following the reforms of Ephrialtes and Pericles (460s-450sBC onwards), the thetes of Athens had it pretty good, being able to get a comfortable living wage from serving in public offices and things like serving in the navy. They, of course, also had the right to vote in the legislature, and could elect members of office such as the strategos, at least for their own tribe.

The wages were possible largely due to the tariffs of Athens as an important trading port and their hegemony over the Delian league, the member-states of which paid a yearly fee.

As for why their quality of life was pretty good? Thetes were the rowers of the Athenian navy. The navy was absolutely central to Athenian military, social and political policy. When people are that important to your society, you can't afford to treat them like trash.

Sources: Plutarch's Lives (Pericles), Terry Buckley's Aspects of Greek History, Thucydides, probably stuff I'm forgetting, I haven't engaged in this content directly for some time.