The best replacement for shopify is magento.

The best alternatives to shopify are rated as following:
★★★★★  magento
★★★★★  bigcommerce
★★★★  volusion
★★★  woocommerce
★★★  wordpress
★★★  squarespace
★★★  big cartel
★★  square
★★  wix
★★  etsy
★  storenvy
★  weebly
★  3dcart
★  magento go
★  wordpress for ecommerce
★  opencart
★  godaddy
★  bigcartel
  yahoo store
  zen cart
  prestashop
  paypal
  prostores
  big commerce

Your alternative for shopify is not listed? Suggest!
Speaking about shopify, one probably shouldn't forget about quickbooks.

Other search terms related to shopify and their ratings are:
★★★★★ quickbooks
★★★★ stripe
★★★★ my own domain
★★★ squarespace
★★ mailchimp
★★ authorize.net
★★ wordpress blog
★ xero
You have in mind a companion for shopify and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of shopify in all years. Click on graph for more.
shopify
magento
bigcommerce