A relevant, authority site. An authority site is one that you would honestly recommend to others. It's a name brand in your industry. It's very well designed and has a very good navigation. Page rank, Alexa ranking, all of that can be spoofed. When it comes down to it, the best of the best will survive the test of time without having to resort to black hat SEO.
I agree with CSCgal, time is really the key. You can use alexa and google PR as a barometer to see if your SEO efforts are affecting the trends but, in link development you really want to select sites that compliment theniche you are playing in. If you develop links to increase traffic you want ones that will augment you keyword(s) density and help you to maximize you position in the organic searchs relevant to your site.