The best examples of nature conservation are hard to put together. “Best” implies that they have gotten the best results and have done something great for those species in danger. In the United States, it takes a lot of red tape to get some real conservation work done. However, as the previous answer states, there are plenty of organizations that are doing well in getting through the red tape and getting work done.
One of the most successful conservation efforts, however, has been in Africa. Instead of trying to incentivize leaving the animals alone, the governments of many of the African nations have incentivized keeping the animals alive on private property. This has helped many endangered animals, including white rhinos.
There are many examples of nature conservation and they range from protecting endangered species to reforestation projects in the Amazon and other areas of the world’s forests on every continent (except Antarctica). Instead of directly answering this question, I would like to highlight the top ten nature conservation organizations. To qualify for inclusion on this list an organization must spend “at least 80 percent of money they raise on actual field work, rather than administration and fundraising”.
It’s so nice to know that contributions from people like you and me are going to good conservation causes. The top ten include: The Nature Conservancy, The World Wildlife Fund, The Natural Resources Defense Council, The Sierra Club, The Wildlife Conservation Society, Oceana, Conservation International, The National Audubon Society, Jane Goodall Institute, and The Royal Society for the Protection of Birds.