The Zen theme is indeed one of the best themes for customizing your Drupal, in my opinion. The theme has lots of information in the page.tpl.php file, the usage of each Drupal function and CSS is included in the page.tpl.php template.
One feature i really like is the ability to have a fixed width of a site, Zen Classic makes it easy, just by checking a button. If you are new to Drupal and cannot decide on a theme, go for the Zen theme and enable the sub-theme Zen classic.
