There are a handful of operas that define the genre. Their time period is irrelevant, their themes go to the very heart of the human condition.They are most importantly marked by the democracy of their relation to the public.The usual distinctions, high and low, serious and comic, popular or specialized, near or far from us in time, manner or society dissolve. We live with them daily without our knowing it. They are the very musical air we breath. They exist in the opera house, on the the concert stage (without scenery), in the recital hall (as excerpts, arranged for piano), in the elevator, on the radio, in the lightest cartoons and the darkest dramas and yes in the shower. The aged remember them nostalgically in good, best and better performances; the casual listener usually remarking after hearing the tiniest fragment from the work remark (ironically) " I don't really like opera,but ..." (that's what you think!) Children ask, "What is that song ?"
A short list of such operas might read : The Magic Flute, Die Walkure, Norma, Der Rosenkavalier, Eugene Onegin, Carmen, Aida, Turandot and ... and whatever you like; the only opera you can't leave off the list is Verdi's La Traviata.
The theme beyond that of the nineteenth century "fallen woman" with or without a heart of gold is more generally the outsider and how love and the outsider collide with society. We see refracted in the drama that follows the most laughable of human foibles and our highest aspirations. Normally society will not yield and love will not be denied - Mayhem, laughter, pathos and sometimes real musical transcendence follow. Men risk head and heart for Turandot, the youths of the Magic Flute move through the threat of death to their goal of light, led by love; Brunnhilde and Siegfried will risk the end of the world and the wrath of the gods; Octavian and the Marschallin race against time and social convention and on it goes.