Debian is a pretty safe choice overall but and I’m sure I’m going to get downvoted like crazy but arch has been a fantastic server OS for me for a while. Debian is pretty hands off but I have some pretty unorthodox requirements/hardware setups and the combination of the wiki and such a wide range of packages supported has enabled me to use the hardware to its fullest potential. Also rolling release lts kernel is pretty dope.
Depends on the type of server too. My media server is arch (aur is godsend with all the weird little tools I'm running) but you'd have to be out of your fucking mind to use it for a web server.
Web server is usually Ubuntu server/Debian with virtualmin.