What is the single best tourist attraction you have visited?

We have had a lot of threads about the best/perfect travel destination, so I want to narrow it down a little bit more. What about the single best sight you have visited? As such, the need for a balanced experience falls away. It can be a one-shot wonder like, as many on this sub would argue, the Taj Mahal in Agra (although I would personally argue that Akbar’s and the Itmad-ud-Daula‘s tombs are sublime as well), or one among an ensemble of sights like arguably St Peter’s Basilica in Rome (it is more difficult to argue that it outshines the Colosseum or the Roman Forum; it might be your favourite as a matter of *preference*).