Earliest hook to generate content during user registration
You can use hook_user_insert(), it should be early enough for you.

Technically speaking, hook_user_presave() is the first hook called from user_save() (even before the user is written to the database), but according to the documentation it is "primarily intended for modules that want to store properties in the serialized {users}.data column". More importantly, it is also invoked when a user account is updated, so that wouldn't fit your use case.