"Best" is really a matter of opinion. Here are some related to what you are looking for.

Business marketing social sites include:

Linked In (very popular)
http://www.linkedin.com/

Ryze
http://www.ryze.com/

Facebook
http://www.facebook.com/

MyBlogLog (Less Business Focused)
http://www.mybloglog.com/

Other social sites tend to mix up a lot types of users and are really more about playing some kind of popularity game on its own site. The demographics of many other social sites is not typically best for business purposes as the traffic generally does NOT convert, unless your business is a blog that sells ads or paid search links and needs this traffic and potential readership.

On that note these pass tremendous traffic (conversions from the traffic is another story):
http://www.digg.com/
http://www.stumbleupon.com/