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Decided to check. According to Dogpile, Boston, Massachussetts's is the opera house with the most hits, with Sydney second. Unsurprising Sydney's hits almost entirely refer to it from the perspective of architecture, which is unsurprising because, until the recent refurbishment, the acoustics were generally regarded as lousy.
La Scala, Milan, is, in historic terms, the most significant opera house in the world, and is inextricably linked with the career of Giuseppi Verdi, whose final opera, Falstaff, was premiered there. The French, of course, maintain that everybody in the world will pass the Paris Opera at least once in their life (but then they're French).
The Telegraph, I recall, seem to have an almost obsessive concern with Sydney Opera House, as it has regularly featured in magazine & news articles in various Telegraph publications ever since it was built. My favourite, because it illustrates the decline in British journalists' literacy, was the news story about a yachtsman who had "collided into Sydney Opera House." in any case, as the accompanying photograph clearly showed, he was actually 100 yards short of even "colliding WITH Sydney Opera House."
I wonder what generation we're talking about? I'm 47, and I'd pick La Scala over Sydney everytime? I would agree that it is the most famous opera house in the world that's sited in Sydney Harbour, but that really wouldn't be much a clue, would it...?