Well friend it would be difficult for me to state about which is the best WP plug-in for stats on a website because as you know that there are thousands of plugins present on the web.
But yes, I can help you in suggesting some names of best sources from where you can download any type of WP plug-ins that too at no cost. Some of the best sources are freewpplugins.com, dailynulled.com and wordpress.org for downloading premium plug-ins. Try it.