What it the perfect link?
It would be possible to list very easily about fifty criteria for evaluating the interest of a link.
A perfect link would be an often clicked link, placed for a very long time at the heart of the content of an old popular page with an optimized text anchor on authoritative site with the same topic as yours.
This page must also be ranked on the targeted keyword and receive a lot of traffic coming from search engines.
In other words, it will most often be a utopia for most links you will be able to get.