Cleverbot.com  is arguably the most popular. The newly popular SimiSimi mobile app is perhaps a close second, particularly in Asia. Wikipedia contains both a Category:Chatterbots  and a List of chatterbots . Chatbots.org also maintains a relatively up to date Directory .
As for what “best” really means, the sky is the limit…. Guile3d.com Denise  is probably the best full featured desktop virtual assistant at the moment. There are a number of learning bots available, such as Zabaware – Ultra Hal Assistant . Ultra Hal is also apparently learning continuously in the cloud from Twitter, and can be interacted with on Twitter @UltraHal . @twthal  is another interactive learning chatbot on Twitter by Ai Research (a-i.com).