With this following list you can get a free backlinks with a push of button. If you talk about a quality, remember as part of your natural link building profile, google expect to see a mix of low qualitty and high quality that linking to your site.
Use these on all of your site, at pace of one of them per week. Means, you use all 4 of them per month. With this tricks, the way link velocity remains constants and google sees unending flow of new links to your site often.