The Great Synagogue, also known as Dohány Street Synagogue.
It is the largest synagogue in Eurasia and the second largest in the world,
after the Temple Emanu-El in New York City.
It was an amazingly intricate structure in an ecclectic mix of styles.
More info here:

http://en.wikipedia.org/wiki/Doh%C3%A1ny_Street_Synagogue

Like most of the architecture in Budapest, this is very ecclectic.
Romanesque arches (characterized by being round)
infilled with modified Gothic stone tracery,
an overall islamic/arabic geometric patterns (a certain grand irony there somewhere)
and Russian Orthodox onion domes.
And of course, many stars of David.
But it was exceptionally well done and very charming.