In my experience, the worst zoos in the U.S. that I've been to thus far have been the now closed Bucks County Zoo, the Seaport Aquarium, and the Brandywine Zoo. Although I think these were the worst zoos I have been to, I don't necessarily think they're bad zoos. The Brandywine Zoo wishes to expand and at least has a few animals not found in nearby zoos, like the serval and swift fox. When this zoo grows larger, I'll come back and reevaluate it. The Bucks County Zoo was an indoor zoo that had good intentions, but community backlash and lack of funds caused it to close down. It was only open for three years and it looked more like an exotic pet store more than a zoo. Like Brandywine, it had animals you normally don't see in other zoos like Philadelphia and the Bronx. It had a caracal, fennec foxes, a spectacled owl, and macaws, which are often used as educational or ambassador animals more than animals on exhibit. The Seaport Aquarium is a small aquarium on Wildwood Boardwalk and is not great. I found most of the exhibits livable. The animals were not kept in squalor conditions and had decent sized exhibits. My least favorite exhibit was the alligator exhibit, which had a tiny pool and fake, plastic grass. Even so, I know there are far worse places for animals and the three places I mentioned were not as bad as some roadside piss-pour zoos in the Badlands.
