I just bought parts to build a computer for Blender, a Ryzen 7 3800m and RTX 2070 and 32 gb 3200 ram. I want the best OS for running Blender with that system. I understand Linux is the OS that allows blender to work the best. There are so many different Linux distributions though. Which one is best for Blender 2.81?
If you are new to Linux then go for one with the best support. That would, without doubt, be Ubuntu ... the user base is so large that it is really easy to find help.
There is a dedicated Stack Exchange site at https://askubuntu.com/ if you like to search these Q&A sites.
I believe they use(d) Ubuntu at the blender institute, certainly I have have seen it in some of their videos.
It is also popular among blender users. This screenshot comes from https://snapcraft.io ( Snaps are an app package format, which is officially supported by Blender).
While these stats may be biased (more Ubuntu users will use Snap than other distros), it does show that you would at least have some company.
Once you are comfortable with it, you will find it easier to migrate to other variants many of which are based on Ubuntu, so it is hard to avoid.
Reading this article on best linux distributions for everyone, all but one are based on Ubuntu.
DISCLAIMER: I don't currently use Ubuntu
... but then again I have 20+ yrs experience with linux and like to edit text files. I have used Ubuntu for long periods, over the years. It is probably your best start point.