This is a tough question but good one! I have a few for different reasons. The first lies in the small town of Kawakawa in new Zealand and is the Hunderwasser toilets. Designed by Austrian artist, ecologist and architect Friedensreich Hundertwasser these "public toilets" are pure genius made out of recycled materials, have a living tree growing out of them and have very few if any straight lines in its design. A very unique and incredible sight .
Another favorite is The Venetian Hotel in Las Vegas. I found it mesmerizing and sitting in St Marks square having dinner inside but feeling like it was outside was incredible. The detail in the shops, paintings, architecture was quite something.
My third top place would be The Colosseum in Rome. Visiting this ancient wondrous building you are transported back to the days of ancient Rome and it is quite an amazing sight and experience.