There is absolutely no “best” e-commerce WordPress theme. Virtually every theme in the .org repo will work perfectly well for any type of e-commerce you can imagine. You’ll need a PLUGIN to enable e-commerce on your site, and for that WooCommerce is the most popular [and therefore almost certainly the “best”, unless you can a priori name some feature of Shopify or some other plugin you need — which I defy you to do]. Personally, I think the size of the market is important, and by that criteria, Woo is hands and shoulders the best. Also it’s 100% free and open source. Some themes, might be “WooCommerce enabled”, like Storefront [the base theme from WooThemes], and this just allows easy placement of things like a shoppiong cart, which can also be done on any other theme with just a few shortcodes.
To be clear: ANY theme will do. You can setup a e-commerce site that looks like absolutely anything, and has all modern e-commerce features, 100% for free, 100% secure on WordPress. Don’t waste your money listening to salespeople selling you garbage.