With the fast growing population of Smartphone users, Google has announced as of April 21, 2015, it will be taking mobile-friendliness into account as a mobile search ranking factor and there will be demotions to your site if it is not mobile friendly.
This change will affect mobile searches in all languages worldwide and will have a significant impact in your search results. Google said the demotion will only impact mobile, smartphone friendly, search results and only impact web pages that are not smartphone friendly or misconfigured when it comes to being smartphone friendly.
If your site is not built right, it won't rank well in the smartphone search results in Google.
This means a mobile or responsive site are now essentially mandatory to show up in mobile searches. Speed ranking factors will be coming to mobile pages as well.
Google prefers all sites be responsive websites. However, Google has also made clear that their search algorithm doesn’t care what type of mobile website you have, responsive website or dedicated mobile website.
What Google does care about is that you HAVE a mobile-friendly site and a good experience is given to the users. They have little preference between responsive, dedicated mobile or dynamic serving, as long as it’s set up correctly.