OpenCart 3.x: Please use Power Reviews, which is a more powerful solution, and includes this function.
This is a very simple extension. It enables a module to show the products that have received the highest number of customer reviews.
Please read documentation.
Support is at http://forum.opencart.com/viewtopic.php?f=23&t=84835
I hope you enjoy this extension. If you'd like to donate, please PayPal to firstname.lastname@example.org