The best site is the one that works the best for YOU.
I just posted on another thread that you should choose to be where your customers are and focus on the ones with the best potential for your business. Pinterest is hot now, but maybe you don't have a business that is very visual. Facebook is one of the biggest, but if you are B2B is it worth your time?
I made this chart that compares the business impact/potential for all the major social media sites that may be helpful for you: