Okay:
Bronx Zoo has the best snow leopard exhibit (at least in the US), best red panda exhibit, best gorilla exhibit, best okapi exhibit, best mandrill exhibit, best red river hog exhibit, best Wolf's monkey exhibit, best gibbon exhibit, best Silvered Leaf Monkey exhibit, best Butterfly Garden, best Madagascar exhibit, best naked mole rat exhibit, best baboon exhibit (geladas), best wild dog exhibit, and among the best tiger, hornbill, zebra, Incan tern, African rock python, Burmese python, Asian turtle, tree kangaroo, bee-eater, argus pheasant, and Congo peacock exhibits. Not to mention the best small rodent exhibit ("Mouse House").
At San Diego Tiger River and the Kopje, and to a lesser extent the gorilla rainforest, are all far superior to what has been built (and designed in-house as mentioned earlier) since those early improvements. The giant walk-through aviaries are spectacular, but are much less exciting than they used to be before they added the freeway-like concrete walkways through them to handle the crowds and wheelchair accessibility.
And the Ituri "Forest" really is an aesthetic mess.