I am sure that all of those listed are must see sights in Italy, and I can think of a couple more which probably should have made the cut rather than some of the choices. Mount Vesuvius along with Pompeii and Herculaneum. especially the sight of the volcano from those two locations.
I went to Verona once for a day when at Lake Garda. The local tour guide who took us around the city didn't even mention or take us to the Roman Arena!, I hadn't even realised there was one until listed. So i'm imagining it as a JY hidden gem.
Mount Etna is certainly the most imposing sight on the list, and one I hope to scale in September - assuming its not grumbling again. Tower of Pisa and the Colosseum are Italy's most famous landmarks. I haven't been to Rome yet, but I found the bell tower a little underwhelming..... and the Sistine Chapel is in the Vatican. As a Catholic I probably should have voted for this, but I didn't 😛, instead to me the most impressive sight was one I was not prepared for and that was the Duomo in Florence.
Emerging from a side street, and there it was towering above me like a sleeping giant. In the city that combines the art, music, culture, religion, education and rich flavours of Tuscany and the rest of Italy. This landmark you have to see, and one I phone almost impossible to get a perfect photo and getting as much in as possible. So close and you don't see its glory and further away many heads get in the way trying to do the same, and I think post cards revert to a birds eye view. So you have to go plan a trip and see it for yourself, because no second hand description or amateur photo does it justice.
Just my opinion.
Chris