What animal that you can adopt from organizations like world wild life fund is an most important to its habitat?

Sharks are a big one as they are a key support in most food chains. Also, if you are looking to adopt or support it can be better to look at smaller conservation groups as often money does not go where it is really needed in larger ones such as the WWF.

If you google "animal species" conservation (example "shark conservation" or "bat conservation" it will give you a list of organizations and groups that you can help support. You can also contact your local zoo or check zoo webpages as many work with conservation groups in the field, many of which are in dire need of support.