What is your favorite e-commerce software package and why?

I have several different packages I use depending on the requirements of the project.

We just launched a new site for 3D Security in Henderson, Texas. They are East Texas leader in high end security systems: http://www.3dmaxsecurity.com.

We built this new site with SEO as a top priority. They are looking to drive more traffic to their website for potential customers in East Texas.

[More]

I have been evaluating ExpressionEngine for some upcoming projects. From what I can tell it looks to be very flexible and powerful. Most CMS engines make it very hard to plug in a custom web design but EE makes it look simple.

What are your thoughts on EE?
What are the downfalls of EE?
Why should I adopt EE as my CMS of choice?

We have been working with Dr. Randy L Morton in Longview, Texas to Search Engine Optimize their website.

Wiki defines Search Engine Optimization(SEO) as: "Search engine optimization (SEO) is the process of improving the volume or quality of traffic to a web site from search engines via "natural" ("organic" or "algorithmic") search results. Typically, the earlier a site appears in the search results list, the more visitors it will receive from the search engine."

[More]