Hi KCZoofan,
I had a similar post some time back, mainly to get ideas for my project, i have visited a number of birdhouses in Zoos mainly, and there is many factors involved i believe. Number of species housed in the structure, Architectural design and style, number of exhibits, history of the place and many other factors.
I have visited some of the great birdhouse of Europe, many already mentioned here and in number of species housed, general layout and history the two that really stand out are Berlin Zoo's Birdhouse building and Frankfurt, with Berlin being superior in species housed there at and having an interesting layout. Frankfurt looking a little more modern and detailed.
But i seriously believe (keep in mind zoological buildings are my passion and especially birdhouses) that none rival World of Birds at the Bronx Zoo. If you place all of the categories together that make a bird building interesting from the visitors perspective Bronx takes it in my opinion. A strong number of species displayed. A large number of exhibits, a walk thru area withing the building, a massive structure by itself, my favorite aspect, naturalistic displays based on themes with excellent art murals in the exhibits, lots of detail and it has been going thru extensive remodeling phases, the last time i was there on Sep 2010, they had just opened the first floor and it looked fantastic, the second floor was still close and it is still close i believe.
The only area that this birdhouse lacks is the Historical Architectural importance, yet that is the very area that makes it unique, the building is very unique.
Below pics of the World of Bird Bronx Zoo