It's not a matter of selecting the "best" but doing EVERYTHING to reach your audience -- email, banner ads, pay per click, social networking such as Facebook, etc. You do all of these strategies depending on your resources
The key is which of these strategies will give you the TARGETED customer. It's one thing to advertise and reach anyone and everyone; but another to actually find those who will actually want to buy from you
Note though that there are two things you need to do:
- reach the targeted audience
- convert these visitors into buyers
Those are two different things.
I suggest you carefully track the quality of visitors each of the strategies you are doing are bringing. Take for example Facebook -- how many visitors does it send your site, and how many visitors from Facebook actually buy from you? What's your ROI? Make sure you use a good tracking software that allows you to study conversion and funnel analysis
The source below talks more in depth of improving conversions and getting targeted traffic to your site