The ideal backlink will come from a site that has a high pagerank, plenty of traffic and uses do-follow links. While pagerank is a complicated and temperamental measurement to quantify, it is easy to find using plug-ins for most web browsers or using websites to look up a site's pagerank. Traffic measurements are easy to find at a variety of sites. One of the biggest names in this business is Alexa. A popular ways of generating high-quality backlinks to sites that fit these requirements is commenting on blog posts.
After you have narrowed down your targets for backlinks, the next thing to check will be whether the links you are placing will be do-follow or no-follow. This is important for climbing the search engine rankings of most popular search engines. Simply put, a no-follow link means that while Google will ignore the link for the purposes of pagerank and SEO. Do-follow links establish authority and credibility with Google and the other major search engines and should be used when possible.
However, no-follow links are not a waste of time. These can still lead to plenty of increased direct traffic, you just will not see the link on your Google Analytics. Ideal places for no-follow links include social bookmarking sites. One viral no-follow link can be worth hundreds of mediocre do-follow links. It is all about quality and knowing how to squeeze traffic through your links.