Curious which of the free themes that come standard with drupal core are your favorites and why. I personally like garland after I tweak it, but its pretty plain. I also like the aquia theme but its not really core. I like using some of the core drupal themes because they don't include a backlink to the developer although I think its important to give credit where credit is do. I also like free drupal themes from drupalthemesonly because of the amount of regions and the no back link, (they lack a large amount of themes but the sites new)
Of all the themes that come with the Drupal installation package, I actually prefer Bluemarine just because it is so basic - throw a default Bluemarine up & create a new page.tpl.php page with Dreamweaver via the Drupal Dream Theme Utility Suite - overwrite the default page.tpl.php, add styles, & I can have a custom, original theme in under an hour.