I don't really think there is a best traffic source for a web hosting site, rather, there may be a best traffic source for YOUR web hosting site. The challenge here is that no one here can tell you exactly what the best source is, that is something you will have to experiment with and try out.
As David wrote, Forums, Blogs, Search Engines (organic traffic from SEO efforts) are likely going to be your best bet.
However, don't forget social media! It's important to have a presence there as well.