The Biserica Mănăstirii Stavropoleos, or the Stavropoleosis Monastery Church has got to considered among the most beautiful churches in Bucharest, Romania. I cannot be certain that it rates as "the most beautiful," although it would have to be a strong contender, exterior as well as interior. I have not visited all 1,883,425 Orthodox churches in Bucharest (one for every resident of the city).
Yes, that number is exaggerated (the number of churches, that is) (as far as I know that is an accurate number for the total population of the city which is bigger than you would think). But there are a lot of churches in Old Town Bucharest, nearly all of them Romanian Orthodox, and nearly all of the them tiny, which would account for the need for large numbers. (The Russian Orthodox church was the only one that looked regular-sized, what you would expect from a suburban Catholic parish church. All of the Romanian Orthodox churches appeared to be, as we would say, fun sized.)
The Biserica Mănăstirii Stavropoleos is incredibly beautiful, inside and outside. Every square inch is ornamentation, including the saints painted on the walls.
Visited May 2019. (That really feels like 10 million years ago as we are now in the fifth month of the COVID lockdowns.)