AWell, it depends. There are many types of links that can be created for SEO purposes. A simple answer would be those that use content to make the links. But that is a very vague answer. The most effective backlinks are those produced through guest posting. While the most efficient ones are those made with social media posts. But each type has advantages and disadvantages. If you collaborate with an SEO agency, then the best backlinks must include guest posting, as it has the highest effectiveness. Also, a professional agency has the manpower and resources needed to also maximize the efficiency of this method.