I’ve only just caught up with this very interesting thread. I am wholeheartedly in the San Diego camp, and reading some of the comments above I am reminded of the (world’s best) film critic, Mark Kermode, who recently opined that the only people who liked the film Hereditary were Those who didn’t really understand horror. Cue, lots of responses from slightly miffed listeners, claiming that they did indeed like and understand horror – and also liked and understood this film.
I might be slightly misinterpreting your analogy - are you saying that people who disagree about San Diego being the best zoo in the world don't really understand zoos? Or, alternatively, that you believe those who disagree with this statement are directing this accusation at those who agree with it?
No, San Diego stands alone. Us nerds talk in hushed tones of Vienna and Berlin, but nobody else does. Whereas if I were to ask my non-zoonerd Australian friends to name the most famous zoo in the world I’m confident some, at least, would come up with San Diego. And with all due respect to a wonderful city, it’s not exactly well known outside the US like New York, Singapore, Berlin and Vienna are either.
There is no zoo on the planet that is more World-Famous than San Diego Zoo.
As with @Dassie rat and @Tim May , I would argue that although the quality of the collection has certainly diminished in recent decades London Zoo is probably still just as world-famous as San Diego.
FWIW, with the exception of Highland Wildlife Park - which I visited as a toddler and hence was always aware of - the first zoos which came to my attention as more than merely abstract "place name + zoo" were London Zoo, Berlin Zoo, San Diego Zoo and Chester Zoo in roughly that order.