What is the best language for back end web development?
Totally depends upon the kind of website you wish to build. If you wish to build a regular content website, you don't even need to write code. You may use an open source cms like WordPress or Drupal or Joomla - written in PHP and the database is handled by MySQL.
Modern website/webapp development is done using rapid application development framework. CrazyEngineers is written using Laravel framework which is a very popular PHP framework. For front-end, we use HTML and Tailwind CSS and jQuery. Some parts of this site make use of VueJs.