Houston Zoo. It's 65 acres, so i don't know if that's a medium or large zoo, but it's the largest I've ever been, I think.
It's changed so much in the last few year's. Many of the old, kinda iconic building are being replaced. The zoo has a new CEO, and he is changing the entire zoo to be more educational and a little more interactive. If you go to the Houston Zoo website, you can find the entire plan that he wants to do.
So far 2 section have been redone and now they're doing the 3rd. They finished Texas Wetlands (Whooping Cranes, Bald Eagle, Alligators, Alligator Gar).
They also finished the South American Pantanal (Jaguar, Rhea, Giant Anteater, Tapir, Macaws, Giant Otters, Green Anaconda, and some monkeys).
Right now they are building the Galapagos Islands and Oceans and also the new bird section. They're gonna supposedly finish the bird area thisbuear and open it and the Galapagos and Oceans section next year.
Here's the link to download a PDF to see the whole plan and what they will be doing:
https://www.houstonzoo.org/about/strategicplan/
Now, other zoos I've visited are the following:
(Mexico) Irapuato Zoo - a very, very small zoo. But, I like it. It's not the best, but because many Japanese people have been moving to the area, they only recently have begun to fix it up again. It's been a few years since I've been, so it might be even better now
(Mexico) Leon Zoo - I've only been once. Leon Zoo is nice, but I prefer the Houston Zoo. They did have a little safari, but it wasn't much. They're part of the AZA, I think
(Argentina) Córdoba Zoo - I went many years ago. I don't remember much, but I do remember that they had temporary white lions at the time
(Houston) HMNS - yes, this is the science museum, but they have the butterfly section, which is an entire area with an indoor rainforest filled with butterflies. They also have a few vivariums with fish and amphibians and reptiles. And inside the same building there's a gallery to view different bug species, such as spiders, centipedes, insects, etc.
(Houston) Downtown Aquarium - I guess this technically would be a zoo. I don't really enjoy coming here much, though, because it's exactly the same. The only exhibit I truly enjoy every time is the white tiger exhibit
(Galveston) Moody Gardens - if you've heard/been, you know why. 2 of the pyramids are revolved around animals. 1 is an aquarium, very nice. The other is a rainforest with monkeys and birds and some pools with stingrays. I've been twice
Edit: I just reread my post. It's NOT alligator gardens. It's alligator gar. I fixed it. We don't have a garden of alligators. Although that would be fun