you can use google analytics for best visitor tracking, still it;s not providing real time visitor tracking so you should go with statcounter... i prefer statcounter rather than analytics.
Analytics is free and as freelistfool said, it comes with tons of features that allow you to track nearly anything you would want to track. Most hosting companies also provide complimentary tracking services but they're not as good as Analytics.
though Google analytics is good tool ,it's not very much accurate Also Google adsense stats for impressions on a single page differs from that of analytics of that same page