Quality, authoritative, trustworthy inbound links (those with minimal outbound links on the same page will pass the most authority as PR is diluted between outbound links unless no followed but that is definately a question for another topic)
PR matters for only those that interpret it right (i.e very few people apart from experienced industry experts), for everyone else that doesn't understand PR (which believe it or not also includes about 95% of SEO companies as well), I would say it doesn't matter at all, forget it - it's a worthless value.
If you are just trying to increase quality of traffic and/or sales to your niche website, do not worry about PR in the slightest. I wish Google would do everyone a favour and disable the visible toolbar PR which is no true reflection or real-time value of a pages worthyness anyway. It just misleads the majority and causes seo scams, mass amounts of paid links and artificial link sourcing/building. It's an aspect of the website marketing industry that has gone barmy - and Google are to blame!