I am getting confused by the following traffic metric systems: Google Analytics, Alexa, Quantcast and Compete metrics because they each show different values. ie. unique traffic, visits, pageviews, etc. Which among them would be the most reliable and why does each show different values.
Google Analytics would be the most reliable.
I was being a bit lazy for not posting this, earlier, I realize that this comparison might help others (including myself!)
A non-biased version is provided by Avinash Kaushik on the differences between these and other systems. I would highly encourage you to go through this post, because I cannot explain things as clearly and as indepth as this man. He rocks!
In short, if you want to measure metrics about only your site, then Google Analytics is the best tool (I could be biased since I have been using this for the past 3 years on a daily basis and am generally able to find answers about data discrepancies if any).
Do note that GA does not provide competitor data, where as some of the other systems do. You can find such data using Google Trends for Websites. Do read a longer post on KISSMetrics on this.
If you really need to know your traffic as accurate as possible:
Make sure you have Google Analytics integrated and running on all your pages or even better: Install some serverside tracking.
Apart from the 3 sites google analytics,alexa and compete,here is another site which will also be important to measure stats of your site and other sites. So, I recommend you try SimilarWeb. The site relies on an information coming from monitoring tens of millions of different browsers plug-ins and apps, creating one of the biggest panels in the world for web measurements. The site gives you an analytic traffic report for any site in any country, and its very easy to use and understand. If you wish to get the clearest picture of your site and your competition I suggest you use this tool. Best of luck!