This is a very short yet powerful read – more of a fictional essay than your typical short story. Le Guin describes an almost perfect society whose well-being comes at a cost. Some main themes are: utopias, utilitarianism, “the trolley problem” thought experiment, moral responsibility, inequality, capitalism, exploitation and slavery.
Full text: The Ones Who Walk Away from Omelas
High-quality audio version: OMELAS – (AUDIOBOOK with full text)
A good analysis: The Horror of Utopia | The Ones Who Walk Away From Omelas
Excellent BBC podcast discussion: Free Thinking – Ursula Le Guin and The Ones Who Walk Away from Omelas
I also recommend checking out this story, which is a direct response to Le Guin: The Ones Who Stay and Fight by N. K. Jemisin (2018)
And here is a classic older story that probably inspired Le Guin: The Lottery by Shirley Jackson (1948)