Here's a list with some of the popular GNU+Linux distros for 2021. More info in article:
https://www.techradar.com/best/best-linux-distros
This can't be in popularity order, haven't even heard of many of these and it's all different types of systems in a mix, everything from servers, IoT to desktop. Interesting read tho.
Debian has to be among the more popular choices, and I do believe Cent OS too is pretty big.