I will give you my opinion based on my personal experience. Facebook and Twitter are very nice and effect social networks but only if you use it on proper way. If you don't use for business than the best network for hanging out with people is Facebook. When you use it for the business,I also recommend to use Tumblr,Pinterest,Instagram,LinkedIn. It won't be mistake if you try this social networks.
Twitter is better for constant stream of comments in short bytes. Facebook is better for presenting the "entire story". Which is better for you will depend on your situation. In our experience, a steady stream of bytes on a consistent basis is more effective than one large post one time. Hence, Twitter is more effective for our needs.
Its totally depends on your way of using example if you are asking for personal use than the Facebook is best and if you are asking for business promotion purpose than Twitter and Google+ are much better for creating quality traffic.
If you are looking to generate exposure and traffic you can try to go beyond the obvious also, here are some good suggestions of where you could extend your reach, for your particular message or product. Get to know each tool however because they each have a purpose and unique success: Facebook Twitter LinkedIn Google+ Tumblr. Posterous Plurk BibSonomy Identi Quora del.icio.us friendfeed bebo Hope some of these ideas help further your success!
if your are asking for just one . i would say ....Just FACEBOOK following twitter, Google+,linkedin ,pinterest and orkut.