The San Diego Zoo in Balboa Park is one of the largest and most progressive zoos in the world that opened in 1915, with over 4,000 animals and 800 species. The Zoo is operated by the non-profit Zoological Society of San Diego on 100 acres of land leased from the City of San Diego. The San Diego Zoo is one of the world's few major zoos to have almost all of its major exhibits be open-air; in fact, the only major exhibition building on grounds is the Reptile House.
The San Diego also operates the Wild Animal Park, which displays a more expansive animal extension. The Zoo and Wild Animal Park regularly transfer animals back and forth due to their species survival plan. They also transfer animals to other zoos around the world for purposes of education and breeding. The Zoo is extremely involved in conservation and species-preservation efforts.
The San Diego Zoo is one of the world's few major zoos to have almost all of it's exhibits be open-air; the only major exhibition building on the grounds is the Reptile House. The San Diego Zoo has been a pioneer in building "cageless" exhibits.
I think the San Diego Zoo by far is the most impressive and world renowned zoo in the world. The only other zoo that would be great comparison is the Australia Zoo in (Beerwah) Brisbane, Australia. Both zoos are very natural and don't feel like they just have animals in cages. The animals are closer to their natural habitat in these zoos compared to others in the United States. I believe that zoos are a major part of humanity due to educating school age kids to help with animal conservation, so the human race will help animal species last longer for the good of the earth. It is just a great place for a field trip and very educational for anyone to learn more.