Generally said best in terms of quality, easy of use/manage, community help. Best for SMC.
When it comes to quality, it almost best. But, difficulty in every aspect and little community help, meaning extra cost of management. This might suitable for middle or big company rather than small.
Relatively good quality in general
- Tomato Cart (web site looks under attack of malware)
Well equipped, newly born. New version of osCommerce.
If you use Joomla, this can be a good choice.
If you use Drupal, this will be the choice.
ref: http://www.webappers.com/2010/07/09/15-best-free-open-source-ecommerce-platforms/
Magento can easily do them all, but has quite steep learning curve, also in the wrong hands Magento can get extremely slow.
PrestaShop is fast and easy, but inside is French. Some of the modules are bit harder to make for the system because of that. Otherwise it also supports everything above.
Joomla + VirtueMart I do not recommend for the moment, since they are quite buggy. Maybe in a few months time it will improve. From the current view I see that PrestaShop has taken away quite a lot of popularity from Joomla VirtueMart
WordPress + e-Commerce (getshopped.org) – is easy to set up. Main difference between Magento/PrestaShop/VirtueMart is that you can put your products literally anywhere on your site (post, page, widget, etc…) thru use of shortcodes. Downside is the lack of multi currency support and extra plugins are required in order to get the multi currency support working properly.
– added for reference from http://forums.digitalpoint.com/showthread.php?t=2412346