A lot is going to depend on your target audience.  LinkedIn is typically best for B2B.  Facebook is typically good for B2C.  There's really no one-size-fits-all scenario, however.  I get a decent amount of quality traffic from Twitter, and I'm focused on B2B due to the nature of my business.

The best thing to do is spend some time researching where your target audience "hangs out."  And then be present where they are.