h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Tyler, Texas: This zoo is my personal favorite. I'm not a fan of large zoos, so this one is just right. What's the point of going to a zoo, if you don't have time to really watch the animals? I'm not interested in having to move so quickly through a zoo that I don't have time to read the available information. For me, a zoo isn't an amusement park where you run from attraction to attraction trying to shove it all in, ticking off on your map which animals you have seen and trying to figure out which route will allow you to see them most efficiently. I want time to watch the animals do what they do. (I also like it that my children aren't screaming with fatigue by the end of the day.)
I've noticed that at some zoos, the animals aren't very active. I'm not just talking about nap time, but in general. Some zoos seem to have perfected the art of making habitats so uninteresting to the animals, that they have no interest in getting up and doing anything. Tyler's zoo seems to have found what each animal wants, while positioning the viewing areas for the visitors in such a way that you really do get to see a lot of different behaviors. The animals look happy. Obviously, size isn't everything.
And.......the zoo is free. Some animal loving wealthy person way back when set up the trust for the zoo with the stipulation that admission never be charged. There are donation boxes scattered around, but admission is free.

Reid Park Zoo in Tucson, Arizona: Second favorite zoo. They have large Galapagos Island type turtles. As if that isn't enough, it is a small, inexpensive zoo. My favorite kind. Their animals seem to produce lots of young, so there are always various animal children to watch. If the rhinos are inclined to hang out near the rail, you can touch their backs. If the giraffes are so inclined, you can feed them by hand. This zoo is actually so small that you can eat lunch before you go, and then spend a few happy hours there, having time to see all of the animals. Admission: three something for kids, and four for adults

Sonoran Desert Wildlife Museum in Tucson, Arizona: Not exactly a zoo, but they have a large number of live exhibits. The animals are all Sonoran Desert animals, so they are not animals you would see in most zoos. Definitely a winter season activity, unless you like 115 degree (farenheit) weather. Because the Sonoran Desert extends into Mexico, they have a really nice cat habitat featuring the small cats from that area.
This is one place where the emphasis is definitely on education. The docents are wonderful. The indoor museum exhibits are fascinating, especially if you are interested in caves, minerals, and geology in general. This one, I recommend highly. Admission: about 15 dollars during the winter, and 12 during the summer

Louisville Zoo in Louisville, Kentucky: A pretty decent zoo. It can be seen in one day without hurrying too much. It's a little bit larger than I like. The habitats are decent too. There's nothing really spectacular about this zoo. Even though it's only been two or three years since I was there....nothing about it really stands out in my memory.

Cincinnati Zoo in Cincinnati, Ohio: To be fair, I don't think I've been there in the last eight or so years. I've heard that it has undergone several major renovations since then. I'll probably be taking the kids to see it later this summer, and maybe I'll be amazed by the changes.
But, until that happens, I have to admit that this is my all time least favorite zoo. We visited it once every year or two when I was a kid, and I hated it each time. It's dirty. The habitats were miserable and the animals looked miserable to be in them. There was rarely any behavior of any kind to watch because the animals didn't seem to have any interest in anything but sleeping between meals. Even the monkeys didn't play. I remember that the stench during the summer was nearly unbearable. The elephants had only a hard packed dirt stockade type of area to go outside in, with a slimy concrete pool. If it was too hot, they went into the elephant house. The building itself is beautiful to look at from the outside, but you had to take a big breath before you went in, and tried to make it out the other side without gasping. Not exactly inducive to stop and watch the animals. I've heard that this has changed. We'll see.
Now, this part is really unfair, because Cincinnati can't help it that it is hilly, but this zoo requires some exertion. Many of the paths were steep, and to get from exhibit to exhibit, you were nearly always going up or down. Add a few strollers, or short legged toddler type people, and you've got one miserable day. What will all this lack of fun cost you? Last time I was there: nearly twenty dollars for an adult. I don't know the current prices. I'll try it this summer, and this might be the last time I give it a chance.

One zoo I visited which really stood out compared with my local zoo in Philadelphia (which is a rather good zoo to begin with) was the Audobon Zoo in New Orleans, LA.

I never saw a happier bunch of animals than the orangutans there! They were playing, moving, getting laughs from the crowd...I almost had the sense that they knew they were an attraction. I'd love to take them out and have a few beers with them sometime.

Overall, Audobon Zoo is very clean and the animals have plenty of space to move about.

Probably too late to ever add it in, but let's not forget National Zoo in DC. Free admission, cheap parking (hey, you can take the Metro in, so no parking at all if you'd rather use that), and good exhibits and living areas for the animals. However, do not stand directly underneath the orangutangs when they're moving across the path. National is also one of the zoos in America with giant pandas and, as always, a research program.

Denver Zoo is also a pretty good zoo. It's been a few years since I've been there, but it is definitely a professional zoo.

Skip Cheyenne Mountain in Colorado Springs and go to the Denver Zoo if you want to see a zoo in Colorado. The whole zoo's built on the side of a mountain, but in my opinion, it's not a very exciting one.

Albuquerque (shameless plug time), has a whole biopark going now, with a pretty good zoo going for a medium-sized city, with southwest animals such as roadrunners and lobos (Mexican gray wolves) and some more exotic ones, like white tigers. Also has an aquarium which isn't bad but isn't nearly as good as ones that I've seen in Baltimore, San Diego, and Berlin. And a botanical garden.