One of the best schools of thought when it comes to the type of link you want to earn your client is to think to yourself...even if links were not a ranking factor, you'd want to keep this link.
I've blogged before about how link builders must take their SEO hat off and stop thinking of search engines and think of the users/customers.
A 'good link' has changed over the years and how it's important these days to understand whether a link is ‘good’ or ‘manipulative’, especially in light of Google's Penguin update.
Links are still a major ranking factor and that's not going to change for the foreseeable future but lets pretend for one minute that links made no difference to search engine rankings. Then, have a look at the links you've built and ask yourself the question "would I keep this link?".
This is a difficult question for many but it becomes much easier when you break it down into 3 things:
I've always focused on relevancy and quality when building links but in some cases I would have to say no to this question - Analytics shows no traffic coming from a link.
However, in some cases, I find that very little traffic was received but the link is on a highly authoritative website. Is this therefore a good link? I would then look at what anchor text is being used and if it's natural/in context and relevant then I would say yes, it's a good link.
I'd say the best link is one from a respected/authoritative high traffic site which has good social presence and engages with your target audience and the link drives good quality traffic to your site.