Personally, I was stunned by the Smolny monastery in St. Petersburg. But this may not like to the representatives of other religions. It only proves that the idea of the beauty is not individual, but public and depends on religion, traditions, knowledge, public opinion (which more and more depends on advertising efforts), etc. Architecture is no exception.
But it was there that I understood what the "Florentine syndrome" (or Stendhal syndrome) is. Much later, in Florence (Piazza Santa Maria del Fiore is beautiful, but ... same time partially alien for me), I saw how it works for others. I doubt that modern architecture can have such an effect.
The question needs refining. Religious or secular building? Ancient, medieval or modern architecture? XXth or XXIst centuries? As already pointed out, it would be advisable to focus on secular buildings, to avoid unnecessary controversies.
I think the most beautiful building in this world is Taj Mahal located in the Indian city of Agra. This building made of white marble is often described as one of the wonders of the world, was built by Mughal emperor "Shah Jahan" between1631 and 1648 as a mausoleum for his beloved wife "Begum Mumtaz Mahal". The Taj Mahal is the jewel of Muslim art in India and one of the universally admired masterpieces of the world's heritage.
This is great building, but only one of the numerous variations on the theme of Palladio (Villa La Rotonda). Others can be seen around the world - from the White House to the Russian country clubs ...
Honestly I have been to London all UK Ireland All USA UAE Dubai China Qatar Doha Pakistan Saudi Arabia Others. But Holy Kaaba in Mecca and Mecca Watch Tower is Beautiful and Best. Regards