I haven't been to a long list of zoos (yet) But my list so far looks something like this
1. San Diego Zoo, obviously, lets not kid ourselves here

2. Kansas City Zoo- Love the exhibits and idea that went into each of their exhibits (Except the Tigers of course)
3. San Diego Safari Park- Its been a couple years since I've visited, but I hope to go back soon (once they get their platypus exhibit)
4. Living Desert Zoo- I consider it my home zoo, even if the one below is much closer to me, it got me into zoos when I was in second grade.
5. LA zoo- whats their to say that hasn't been said before? Still have passes though.
6. Sea World San Diego- The faults are very well documented, but where else where I see a Emperor Penguins, Orcas, and false killer whales?
7. Topeka Zoo- Ups and downs, love the tropical dome, but the rest of it was meh. Hopefully camp cowabunga makes it move up the list a little bit.
8. Wright Park Zoo, Dodge City, Kansas- Multiple Tiers below anything else on this list. Don't even look it up. However its where my profile picture came from, and I didn't even have to zoom in to take the picture.