Actually, it all depends on your requirements and the level complexity you want to make it more secure. If you wish to build a proper online store, then I would suggest consider shopping cart applications such as osCommerce, Prestashop, Magento, etc... Though, you can build e-Commerce site with WordPress, Joomla and in Drupal as well.
I would say Wordpress as it's the most used CMS out there and due all the plugins and themes that are readily available are easily integrated with Wordpress. Also, there are many tutorials available online for one to get well acquainted with WP.
More info would be needed. What are your requirements and budget? Some may lack what you are looking for and other more than you need at a cost you may not want.
There is no best. Whatever fits in your budget this is best for you. There are several choices, that you can get from online search. Rather you can consult with a good consultant to discuss face to face to clear your doubts.
It depends upon the no. of products you want to add on your website and your budget for the same. But, in general, Magento is best CMS for an e-commerce website.
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