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.