When it comes to link building, you need to find the best guest posting service there is. Keep in mind that acquiring links is not about numbers. You need a company that understands your website and can generate content ideas that will influence audience in the right way. You want targeted traffic, which will only happen when contents are more inspiring and viral-worthy.
Before you hire a company, check the range of work they have done, the quality of contents they create on an average, and if they are willing to work for a small order, just to test their services.