Google's latest algorithm is the Hummingbird, a complete upgrade of the Penguin and Panda algorithms. Basically, the algorithm takes into consideration each word in the search query, upgrading conversational search mechanisms.
Having said that, guest posting is already a thing of the past, due to the amount of spam seen by the Google's web spam team. So better avoid that. Back links are still the thing that determines SEO, as well as your web design and on-site content.