Links from high PR sites. But don't bother yourself about the pr your site instead think of other techniques that may improve your SERP.
Try to get more and more enough links from Ethical seo process, you will get better results, For short term buying links in Good, but believe me after long time, you will not get any juice from those links.
Quality Links. Content is also a factor. Try to impress most of the authority sites. (news, .edu, .gov) etc.. So they will link to you.
Blogging is the best way to increase PR for your site. and also link exchange with high quality PR site.
Create high quality articles and submit them to ezinearticles or article directories. Be sure to put your links on the authors bio so that when people scraped them your links will be included too.
Backlinks Backlinks ONLY backlinks But why waste time with worthless things like PR .... concentrate on getting rankings ...
I find blogging is a useful way, but it takes a lot of time and efforts and constantly work on your blogs.