The BEST - it depends on your niche. If your niche is arthritis and you can get inbound social links on a health or arthritis site, it may be better for
SEO than just getting a generic digg link.
Also depends on the niche, because you need to be careful you arent too off topic for the social network you are posting on.
Sphinn for instance is mainly good for
SEO related topics. If it's off-topic it will be deleted or buried.
So don't even try it unless it's a fit.
Digg is good for certain things, but diggers are pretty particular and will bury lots of stories.