Hi July,
I'll start with the rule of thumb: The highest quality link is the one that flows in naturally (The one you don't pay for it, give something in return, etc.)
This morning I smiled a bit, while reading a post written by Jim Boykin back in 2005. This phrase was the reason:
"And in Matt’s perfect world of the web, no one would need SEO’s as everyone would just create great content, and people would link naturally, and Google would sort it all out with the best results on top……yea right."
Now, there are several factors you should consider when judging the quality of a backlink. I'll name a few:
- relevance of the page
- age of the domain (older is better)
- number of backlinks pointing to that page (ideally, should be from the same niche)
- on page link placement (a contextual link it's better than a footer or blogroll link)
- number of outgoing links (the lower the number, the better)
- trustworthiness of the page
It might be a good idea to get a link from your friend, sounds like a good plan. He could write an article about you, mention your website in a post he'll write, cite some awesome information you wrote, etc.
Avoid sitewide, footer, blogroll links.