The very best links , that can move you up the serp ladder in a demonstrable way , is what I call a virgin link , it would have a visible PR of 3+ , it need not be related to your sites content ( better if it is of course ), the most important element is the incoming links to that page are very clean , that is they come from sites in very good neighborhoods , therefore google's trust flows through these links to your site , why would these valued trusted sites link to you if your site ( page ) is not valued by them ?
The second equally important point is the linked page will have very few outward bound links ( 1-10 ) no more.
This link would be on an eternal static ( or little change ) page ( even an old fashioned vanilla .html page, all the better ) The problem with dynamic content pages is your link may get buried over time if content changes constantly
These links are hard to get , naturally , but may come from relatives , friends , contacts, or by by good luck or merit -- pure gold
