As the others said, the answer is not black or white. There is no silver bullet and something that worked in one industry might not work in another one.

As an example, I ran PPC for a wide range of company. I acquired free trials for less than $1. In another company, we acquired emails for $15.

In order to discover what can work for your business, you need to run experiment on each channel and discover by yourself. You'll be able to compute the CAC (Cost of Acquiring Customer) of each channel and to discover what is truly effective for your business.

If you want more information on the overall process, I'd advise you to read this article: http://www.coelevate.com/essays/growth-process-first-tactics-second