Bronx Zoo's Congo Gorilla Forest gets my vote:
Congo Gorilla Home Page
Just a phenomenal exhibit-- by far one of the best in the world, with a very large number of lowland gorillas (ISIS inventories 20). Last time I was there, I photographed 5 tiny gorilla infants clinging to mother/riding on mother's back, in addition to numerous toddlers and juveniles wrestling and chasing one another. The rough-housing never seemed to end.
Other species in the Congo Gorilla Forest: Congo peacock (great place to see them very close), Wolf's guenon, De Brazza's guenon, okapi, red river hog, mandrill, b&w colobus, green wood hoopoe...
And right outside the CGF is a wonderful African wild dog exhibit-- with a huge pack of the dogs all interacting, chasing, etc.
The Bronx Zoo is an exceptionally beautiful zoo, as the land was purchased in the late 19th century and the old trees still grace the premises everywhere, making much of the zoo look like an old growth deciduous forest. And yet, the hard core world of the urban Bronx is just beyond the zoo gates. It is quite a shock to ride the freeway from Manhattan past miles and miles of rough-looking tenements, only to find yourself suddenly immersed in the zoo's climax forest of oaks and beech, with migrants singing in the canopy and bison grazing in a pasture as you enter.
The Bronx is deserving of its reputation as one of the world's great zoos.