I think,A good quality link is that which is comes from a high page rank web page of a website pointing to your website. For eg A link coming from a pr7 page is consider good as compare to pr2.
A link on a page that ranks within first 5 pages of results for keywords related to your targeted keywords is a great link PR etc. doesn't matter at all.
If it comes from a high page rank page and that page is relevent your site and the link is completely natural one through G' eyes. It will be even better if that website is a reputable one.
A good quality link, in my opinion, is a link that is relevant to your niche. It should also come from a site with good PR and reputation.
This is what a good backlink should look like: 1. From a high PR site (preferably a high PR PAGE) 2. Uses your keyword phrase in the anchor text 3. It's a 1-way link 4. Comes from a relevant and related site 5. Comes from a high ranking site
A link from non-ghost site, a genuine site that has real visitor. This is what I called a good quality link. Anyway this is just what I think. Other may view differently, other may said from a high pr and relevant site page. But high pr, relevant site, but a ghost site is not a good quality if I might add because someday the ghost site would be terminated eventually.
Same theme site & OO Link from a good pr page with less outgoing links OO A in-content link from a content page & not a links page OO A link from a .edu or .gov website OO A one-way link from a content page
big PR isn't all that is to it, for good SEO you need relevancy, posting on a tech blog for a backlink to your health blog/ on a topic about a car in the tech blog isn't very relevant, for example
Higher PR with niche relevancy and its even better if it is coming from an authority site related to your niche.