Are you asking what platform is the most popular for running one's own ecommerce shop, or are you asking which ecommerce store is the largest?
If it's the first, according to BuiltWith.com
, in 2013 the most popular ecommerce platform was VirtueMart (followed by Zen Cart, Magento, WooCommerce and osCommerce). According to this Quroa thread
from early 2013, popular choices include Magento, VirtueMart, PrestaShop, Zen Cart, osCommerce and Open Cart.
If it's the latter, according to Internet Retailer
, the largest ecommerce store in 2014 was Amazon.com (followed by Apple, Staples, WalMart, Sears, Liberty Interactive, Netflix, Macy's, Office Depot and Dell).
Why do you ask?