Effective - target real sites for your backlinks. Free PR on a good site with traffic and many ranking keywords will be much better than paid post on a dad site.
First you find the domain where you can do the submission, there are having several strategies through which you can start link building. The backlinks which are occurred through free tool are not good, they are the spammy links and it may affect your website by penalizing it.
Link building is not Just creating a Backlink randomly. You have plan and execute it perfectly to achieve good results. You can Build links through Articles submission, Guest Posting, Press releases, Forum Postings, social Media, Social Bookmarking, etc... and then do some research where you competitors are linking to. The try get link from that sites. Backlinkwatch ahref, ranksignals.com opensiteexplorer these tools can you help you to find backlinks of your competitors.