Well, according to "America's Best Zoos" Kansas City has the best. It is located on 100 acres, with 17 acre plains exhibit, 4 acre elephant exhbiit, 3 acre chimp exhibit, 1.5 acre guinea baboon exhibit, 2 acre gorilla exhibit, as well as awesome exhibits for cheetahs, lions, black rhinos, giraffes/zebras/ostrich.
Other average to above average exhibits for these animals: warthog, saddle-billed stork, leopard, bongo, red-capped mangabey, black mangabey, yellow-backed duiker, red river hog, lappet-faced vulture, bateleur eagle, aldabra giant tortoise, red-flanked duiker, blue duiker, southern ground hornbill, an aviary, serval, meerkat, black-footed cat, bat-eared fox, black-throated monitor, rock hyrax, masked lovebird, bushbaby, slender-snouted crocodile, kori bustard, secretary bird, kori bustar, leopard tortoise, hippo, and wild dog.
Other contenders for the title according to that book are
SDWAP
Brookfield
Busch Gardens
Dallas
DAK
Honolulu
Jacksonville
North Carolina
Toledo