Apache
Responsible for serving a large majority of web server traffic, the open source Apache httpd is highly configurable and modular, and well documented.
I worked with Apache for my web server instances for many years to great success, especially when developing dynamic content sites on LAMP stack through the mid 2010s.
Apache has made strides in performance and features in recent years to compete with Nginx, though the latter has several benefits when working with a JavaScript stack.
Apache remains a vital skill for managing legacy deployments.