Google Analytics
There are dozens of analytics packages out there and it's hard to beat Google's free suite which is simply called Google Analytics. It's easy to install, quick to get started, and has easy-to-read charts. Yet, it boasts very powerful features like advanced segmentation, customizable reporting, and industry benchmarking.
When using Google Analytics with Drupal, it's even better. It's easy to install, configure and test thanks to the Google Analytics module.