What is the most popular tech stack for a website with back end?

Err, hard to say exactly. Based on what certain CMSes use, probably either a standard LAMP stack (PHP + MySQL on an Apache server with a static front end) or some sort of JavaScript based solution with NodeJS on the server and a framework like React/Vue/Angular on the front end (like the MEAN stack).

The latter is because it's what many 'trendy' companies are using, especially startups and those inspired by them, and the former because it's what most 'off the shelf' solutions are built in. Like WordPress, Media Wiki, Drupal, Joomla, Magento, most forum software, etc.

Those are probably used for about 90% of websites/web apps out there.

I believe wordpress is behind like 40% of the web.