Wow - that is hard to say.
If speaking of historical monuments, touristic attractions, etc., ancient or more modern, there are quite a few, for their beauty or for their weirdness.
Some of the "must see" buildings:
http://www.ranker.com/crowdranked-list/the-most-beautiful-buildings-around-the-world?var=12&utm_expid=16418821-253.BAWaw7_5T3qAVTf7EdWDtA.1&utm_referrer=https%3A%2F%2Fwww.google.ch%2F
Personally, I liked cathedrals, museums, castles and other famous historical buildings, for example Sacré-Coeur, the Sydney Opera House, the Forbidden City, Saint Mark's Basilica, Notre-Dame de Paris, the Brandenburg Gate, not to forget the Golden Gate Bridge or the Eiffel Tower, etc.