Which authentication to be used when using Django Rest Framework and IOS app?

When you are using Django REST framework with iOS, unless you are using a browser, the standard Django authentication system is out of the question. This is exposed through the DRF authentication system as SessionAuthentication and it relies on your application being able to transfer cookies and the CSRF token with the request, which typically isn't possible.

In most situations where you are using the Django authentication system already, and you can trust your app storing passwords, you would use something like BasicAuthentiction. Most people can't though, or they don't trust their application ecosystem, so they use a token-based authentication system like TokenAuthentication or OAuth2Authorization (in combination with an OAuth provider). You can read more about each authentication type in this answer on Stack Overflow.

But in your situation, you are basically restricted to just using something like OAuth 2. This is because you need to associate a user with a token, and most authentication systems require you to provide a username and password. For social accounts, this usually isn't the case, and they would not normally be able to log in. OAuth 2 works in combination with the standard Django login, so you are not restricted to just a username and password. I've written more about how this works in this detailed Stack Overflow answer.