For Columbus and San Diego, I put Columbus first for a few reasons. Firstly, I just have more recollection of Columbus than San Diego, mainly because I was much younger when I visited San Diego. Second thing, Columbus was a bit easier to navigate. San Diego wasn't so much "hard to navigate," but once I left, I figured out there were certain areas I passed right up. Throughout my visit as well, that was really my only problem with San Diego, but if I had studied the map more, I would have probably been able to navigate it much easier. Columbus, on the other hand, basically felt like multiple "loops." You didn't really miss out on any exhibit, and everything was much easier to navigate. The last thing was the theming. Both zoos have phenomenal theming, the best I've ever seen in zoos, but I do prefer the Columbus Zoo's theming. It made you feel you almost were in the animal's native lands, and the exhibits seemed to be a bit more informative. San Diego, by the way, though, has incredible theming as well, and like Columbus, it almost feels you were in their native habitat. The main difference is from what I remember from San Diego, as well as the exhibits were themed, they seemed to all be similar somehow, and it felt a little repetitive. . I feel that once I visit San Diego again, it may be my favorite, but for now, I put Columbus, but San Diego is very close.
Now, to the Cincinnati Zoo & The Bronx Zoo. The Bronx Zoo is my home zoo, and has always been my whole life. My family and I went every year from when I was just a baby until I was 8. After that, I probably had 3 other visits until now. I'm going to be heading back next Friday, and I'll post a detailed review on the zoos forum. But for now, I have Cincinnati at my #3 spot. One reason for this is the layout. I love large zoos, and I'm sure everyone does, but with the Bronx Zoo, the exhibits are too spaced apart. Sometimes it can take more than 5 minutes to walk from one exhibit to another, whereas at Cincinnati, the exhibits are spaced, but to a point where they are still only a minute walk. The other thing is the Bengali Express Monorail. A monorail ride is always fun, and I love the Bronx Zoos, except for one big thing, and my main reason for putting Cincinnati first. The monorail closes, and the animals are not on exhibit for 6 months of the year. I'm putting that in bold because that has always been something that has annoyed me since I was still a child. There is absolutely no way to see any of the Wild Asia animals, including their Asian Elephants (which is my favorite animal, along with their African counterparts, and what I enjoy studying the most), Sumatran Rhinos, and Przewalski's Horses. I'm not sure what the reason is, maybe it's the animal extremists going after Happy still, or maybe it's something else. It's still a shame for only half the year, you can see those exhibits, and even then, you only get a limited time at each exhibit, and you would have to re-ride the monorail again. But Cincinnati and Bronx seem kind of similar. They both keep their theme from the time they first opened, with both of them having former exhibits and buildings as relics, or revamped into new, really well-designed enclosures. Both of them are also very natural, with a bunch of trees, bushes, and flowers, making them both like a botanical garden as well. (Even though Cincinnati Zoo is a botanical garden as well.)
Every zoo on this list, though, I truly did enjoy, and these top 4 will always be very close, and may change in order through the years as well. I plan to visit a good number of zoos this year too, and once I get to more places, I'll definitely update my rankings.