Hmm, well I have been to the Point Defiance Zoo and Aquarium in Tacoma, WA; the Woodland Zoo in Seattle, WA; the Los Angeles Zoo, the San Diego Zoo, and the Boise Zoo in Boise, ID; alongside my local zoo, the Oregon Zoo in Portland, OR.
Overall, I have to say is the Oregon Zoo. They have mostly geriatric animals and are well known for their geriatric care, as well as their Asian Elephants, which are cool, but not the big sellers. What makes my favorites, are two things. 1) they have an amazing collection of native species, and 2) They used to have Amur Leopards, and still have Amur Tigers.
And there is something about seeing native species. You don't see cougars in the wild, they see you. They aren't called ghost cats for nothing. California condors were wiped out of the area in the fifties or sixties, same with the Sea Otters. They had Steller Sea Lions until the last one died a few years back (At the time of their deaths, they were some of the oldest ever recorded in captivity, like many of the Oregon Zoo's animals).
I think a lot of zoo put a huge emphasis on exotic animals, especially those from Africa, leaving people not knowing about the species in their own backyards, that are directly impacted by their action in their everyday life.