There are really no good, better, or best links to obtain in my opinion. There are only bad ones you want to avoid.
A link is a link is a link. Sure, you can have an authority site with a high PR of 8 or 9 pointing to your site but is that one link really better than 10-15 relevant links from low or no PR sites?
Quality and quantity are BOTH important. While some obsess over getting certain PR links to their site, it is a focus to waste too much effort on. If you come across a good site that would be a good candidate to get a link from, by all means go ahead and get it. But spending all the time in the world to do it is another matter.
Just consider this from a search engine point of view. If a relatively new site with a PR0 suddenly got a bunch of PR 5s, 6s, and 7s to point to them, it would look pretty unnatural. Or if this same site suddenly got an influx of 50-100 inbound links when historically it has averaged about 5-7 a month, it would look unnatural.
A good rule of thumb: If you think some tactic you use looks unnatural to the search engines, it probably is.
If the search engine considers it unnatural, chances are you and the other site owner could be hit with penalties in your PR, be de-listed from the search engine index, or even sandboxed. An article about the Google Sandbox will be covered in more extensive detail in a later installment.
A good site will have a variety of similar topical sites pointing to it from lots of different servers. This is the most natural looking and one that will give any site longevity in search engine rankings.
Remember, it’s about relevancy, not Page Rank. Relevancy is the major key to the future of rankings.