best os for mac mini 2012

Catalina 10.15 is the highest supported macOS version for a 2012 Mac mini.

If your 2012 Mac mini has 8GB or 16GB of RAM and a healthy Hard Drive, then it should have no problem running 10.15 Catalina.

How to download macOS - Apple Support

Your best bet for success, is to only use the prescribed macOS versions and stay clear of, unsupported versions and third party hacks.

Remember that when you leave Mojave, older apps will stop working. Catalina and higher require all apps to be 64-bit. No wiggle-room

Office 2011 and older are 32-bit cannot do Catalina. Office 2016 can, but only if already updated to 64-bit. Office 2019 and higher are 64-bit capable but MS only supports the current and two older mac OS versions. YOu cannot install Office on Catalina any more.

This free tool will help you find what apps are too old and advise whether upgrades are available:

https://www.stclairsoft.com/Go64/index.html