hmm... the best for what ? design ? you can make or order a design template for all of kind of these cms. seo ? there are a lot of plugin to help seo for these cms. successfull sites need proper marketing method. imagine you make great design but no sales made, or your design is standard but you're busy to process orders.. which one will you choose ?
Magento is an awesome shopping cart for an eCommerce website. It's easy to use and the store can be easily customized and optimized.
Os Commerce is better than Joomla when it comes to ecommerce website CMS but Magneto is leading in the race with everything that is needed for your ecommerce site. Whichever CMS you choose, do make sure that the website is easy to navigate and the pictures are clear enough for the people to understand. Also make sure your payment gateway is secure and you have a shopping cart for maximum convertibility.
There are several good eCommerce open source free CMS available. You can choose one according to your requirements.
SHOPIFY - The main reasons I prefer Shopify is because they do not charge a “per transaction†fee like most big eCommerce CMSs do, they have an “App Store†that you can log into from your account where you can add all kinds of great features to your store with the click of button, they have a free iPhone app that you can run your business from and most importantly you can build an online store that looks amazing and that CONVERTS VISITORS INTO BUYERS without paying thousands of dollars to a webdesigner.