I think to understand which is "Best" you need to understand what "Web 2.0" is. Web 2.0 by definition is at its core the difference between static and dynamic.
I would go so far as to say that Web 2.0 is not determined on a Site level, but on a Page level. A home page that is static would be considered web 1.0. A Squeeze page that does not have a counter or the like would be considered web 1.0. A blog page that does not have ratings, or commenting could be web 1.0.
The premise of the question "Which Web 2.0 site is best to increase the site backlinks & traffic in less time?" really doesn't mean anything other than which dynamic site of all the sites in the world is the best to get back links and traffic.. the answer would be the main page of Google - right there when it says "Google" and has 2 buttons "Google Search" and "Im feeling lucky".. to have a link right there... life would be good.