I'd say the golden age of the Bronx Zoo was in the early 2010's. The monkey house and World of Darkness were still open and Madagascar had just opened, and I think maybe the Rare Animal Range was still open and emus were still at World of Birds. Good times.
A. World of Darkness and Rare Animal Range were closed by 2010.
B. Madagascar! opened in the late 2000's.
C. Emus are still at World of Birds.
It's hard to pinpoint the "golden age" of a zoo such as Bronx, especially when their exhibitry has always been so revolutionary. While the late 2000's may be when they had the most exhibits open at once, I might be more inclined to say the 90's into the mid-2000's were the zoo's best age. They could still afford to pay NYC's ridiculous taxes to open new amazing exhibit complexes such as CGF and Tiger Mountain, JungleWorld and the Wild Asia Monorail were still bursting with diversity (including Proboscis Monkeys), the zoo was arguably the best in the country for small mammals between their world class nocturnal house, superb New World primates house, and unique Mouse House, the zoo had just saved the last Kihansi Spray Toads from extinction, they were helping set up the brand new Sumatran Rhinoceros breeding program and hopes were high with the arrival of their first female, and the zoo was still doing amazing work at the Wildlife Survival Center on St. Catherines Island. Sadly, being either not alive, an infant not living in the northeast, or far too young to remember depending on the year, I have next to no memories of this timeframe. Thinking back on the zoo's history, though, other than their early years when they had animals such as African Forest Elephants and Thylacines, I think this era would be the one I'd like to visit the most if I could choose. The zoo's exhibitry was peaking and the collection was probably about the largest it'd ever been, can't get much more golden than that as far as I'm concerned.
I do like to think that the zoo's "peak" has yet to be reached, however...
~Thylo