Naturally, I have to answer Drupal. While it hasn't seen the level of activity a more targeted community service like Stack Exchange generates, Quora still generates a fair number of questions about Drupal and about CMSes and eCommerce in general. I've seen several seasoned Drupallers around answering questions, including Dries himself.
I'm still a fresh face at Quora, but this question was easy enough to answer. It's funny to think about it in such terms, but I've spent over a fifth of my life developing eCommerce solutions on top of Drupal. For the past three years at Commerce Guys, I've helped make the public case for our vision to make Drupal the number one open source eCommerce platform in the world. I think we're on our way.
I'd hope that many others in the community would be able to make a strong case for Drupal as the best CMS for eCommerce, so I've reprinted the bulk of my answer here. If you haven't thought along these lines before, maybe some of the strengths I highlight can make it into your next pitch! ; )
Drupal is an incredibly powerful and flexible CMS, used for sites ranging in size from simple blogs to The White House and for sites as highly trafficked as GRAMMY.com on the night of their awards show. Its strength lies in its community, which includes thousands of active developers around the world working together to make the solution better. Every contributed module is freely available on drupal.org, including the two eCommerce projects I've worked on - Ubercart (which took an "application" approach to eCommerce by porting the osCommerce feature set into Drupal 5) and my current project, Drupal Commerce (which takes a "framework" approach to eCommerce by defining the basic data models and systems you need to do commerce on Drupal 7).
Now, why turn to the community and highlight free? In my opinion, the nature of Drupal's licensing and community governance is a huge win for anyone using it to develop eCommerce features and sites. Developers are incentivized to collaborate, not develop silos of functionality that they can sell for a few dollars (to a few hundred dollars) a pop. Drupal Commerce didn't have to define its own Views engine for listing products, line items, orders, etc. - it just integrated with the already existing Views module. Drupal Commerce didn't have to write its own conditional evaluation engine to power pricing, taxes, discounts, etc. - it just integrated with the already existing Rules module. Instead of silos, we have fully interoperable contributed modules all built on the same base Drupal 7 components. Each new module that comes along simply uses the underlying API and can suddenly be used alongside existing modules without those modules ever having to know they exist in turn.
This results in a fair amount of "Aha!" moments when you realize the next feature you need to add to a site already exists. By way of example, on my Drupal Commerce powered cheese site, Real Milk Cheese, I wanted to offer a discount to anyone who connected via Facebook. I didn't have to write Facebook integration or buy a module from someone - I simply grabbed the FB Oauth module, which happened to have been already created by a friend, and integrated it with Rules to use FB connection as a condition in my pricing engine.
There are a fair number of hosting options available tailored specifically to Drupal, with Acquia Dev Cloud and Pantheon being two of the leaders, and Commerce Guys preparing to enter the market via our Commerce Platform. These services are built on PCI compliant cloud providers, but there are also a wide variety of payment gateway modules already integrated that offer PCI compliant payment through redirected / tokenized payment solutions (think PayPal, Braintree, Stripe, etc.).
Last but not least - Drupal is first and foremost a CMS and community publishing tool. It excels at search engine optimization and offers a wide variety of community features, enabling you to reach and engage your potential customers. As expected, there is even a Drupal specific internet marketing company called Volacci (ran by another friend from Austin - see, we're all very friendly ; ) that can help further optimize your site and draw a crowd.
Commerce Guys has a very aggressive strategy to court and integrate new service providers. The Drupal market is huge, and eCommerce players want to have access to this market. Look for us to continue to build out the offering in the Commerce Marketplace, which will enable you to easily find and enable integrations with payment, analytics, e-mail, and other services providers.
We think it's much better to turn a CMS into a full-featured eCommerce system than it is to try and tack on a CMS to an eCommerce application. By the last reckoning, over 20,000 other people agreed (cf. http://drupal.org/project/usage/commerce).
You want your best SEO, performance, and flexibility driving eyes to your product pages and engaging customers there, not on your blog posts and policy pages.