New website using open source CMS

Looking for new website shell to be developed in an open source CMS. Open to suggestions on which tool (and why). Needs to have three basic lines of business able to be setup along with general company information. In each area need to be able to setup specific product offerings/capabilities and so forth. Would be nice to incorporate some tasteful flash intro's but not critical. Must be template drive so that end users can add content over time. SEO would be desirable. Previous sites you've developed would be desirable to see the quality of your work. ** Update - would like to refine our preferred open source CMS to Drupal 6.X **