Not all links are created equal. I think everyone knows that. But few people consider all the factors that go into a strong link.
The obvious ones:
Page Links - Does the link page have lots of links to it?
Pages with lots of links are valuable. Look for pages with internal (site owner thinks it's an important page) and external links (the rest of the internet thinks it's an important page too) pointing to that page.
Outbound Links-Does the page have many outbound links?
Pages with lots of links out dilute the trust passed through each link. Look for pages with only a few outbound links. Make sure they point at trusted sites.
Indexed - Is the page indexed?
Check if there is a Google cache and how old the cache is. This gives you an idea of how often Google crawls the page. If it's not in the cache it's probably not worth the effort.
Page Age - How old is the page?
Pages that have been around for a while have a lot of trust. Check how old the domain is and if that page available in WBM. Old pages are great.
Anchor Text - Does the link have your keyword in the anchor text?
Word of warning, in your site's overall link profile, anchor text links should make up a small number of total links.
I skipped page rank, because if the page meets the above requirements it probably has a pretty good PR and these factors are more important.
Now some less obvious ones:
Relevance - how close is the topic to your topic?
The ideal page will have your keyword in the title tag and throughout the body copy.
Placement - Where is the link located on the page?
A footer, sidebar, header or comment link isn't as valuable as one from the main body copy of the text. Look for words like: links, ads, sponsors etc... if they are above your link followed by a list, odds are the link value has been discounted. Links pages are worth even less.
I'm not saying that links that don't meet these criteria aren't valuable, just that links like these would have more value.
links from the main body copy of old pages with few outbound links and lots of inbound links are really awesome.