The best replacement for ubercart is commerce.

The best alternatives to ubercart are rated as following:
★★★★★  commerce
★★★★  opencart
★★★★  virtuemart
★★★★  drupal commerce
★★★  zen cart
★★★  commerce kickstart
★★★  magento
★★  prestashop
★★  commerce drupal
★★  oscommerce
★★  drupal ecommerce
★★  ecommerce
★★  e-commerce
★  zencart
★  e commerce
  e commerce drupal

Your alternative for ubercart is not listed? Suggest!
Speaking about ubercart, one probably shouldn't forget about drupal 7.

Other search terms related to ubercart and their ratings are:
★★★★★ drupal 7
★★★★ drupal 6
You have in mind a companion for ubercart and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of ubercart in all years. Click on graph for more.
ubercart
commerce
opencart