Most SEO Agencies should know to surround their websites as well as their clients with high-quality contextual links, you win pretty much every time!
When you look at high authority links on media and other sites & guest posting… note this is normally a pricey exercise for higher budgets.
I would create small/mini authority sites/properties on blogging platforms linking back to your website, with the right anchor text mix.
Brian Dean has also written an in-depth article on SEO Link Building, a great read if you have time. I could not have explained it better myself (no really…)
Relevant contextual links driving your rankings upwards
Article syndication by other sites gives you authority
This service also helps with increasing your domain authority