I'n not sure why the san diego zoo is so highly rated unless you love hiking on hard cement up hills with pollution and noise from buses, huge crowds, and then look at SOME of the animal enclosures where you can barely see most of the animals due to how large and badly designed the enclosures are just like the bronx zoo. Way too much walking in between exhibits. I guess if you're into that type of thing. I like to actually get close to the animal. I think they're missing the fact that the animal is being kept in captivity for our enjoyment and there's nothing wrong with that. I think many local neighborhood zoos are better than the big ones listed in your poll. Smaller enclosures well designed so you can actually see and get close to the animal(I'm not talking road side hell hole size cage, but not a massive enclosure either), less crowds, less expensive, no polluting buses, and softer ground to walk on at some zoos.
Southwick zoo is way better the bronx zoo or san diego, but wasn't listed in the poll. I chose Cincinnati zoo in the poll.(didn't see the option for "other")