best os for 2010 imac

You will get a variety of answers and opinions to that question.

What is "best" depends on what you want out of your system and

how secure you want it to be.

In terms of secure, the newest macOS that can be installed on your machine is

High Sierra and is still being supported with security updates. How much longer

that will be is up to Apple but past rule of thumb has been two versions behind

the current.

What is also "best" depends on you apps and any hardware attached to your system.

If you are on an older macOS, some apps or hardware may not be compatible with

a newer version and may require upgrading to be usable.

There was a mention of Linux in a post, but unless you are basically a computer geek

and willing to sacrifice the use of some applications you use and spending a lot

of time searching for alternatives, if any, it will likely not be for you.

High Sierra, the most up-to-date OS a 2010 iMac can run. The most current version is 10.13.6. Your iMac should have a minimum of 8GB of RAM installed, however it is recommended to have 12GB or more. High Sierra is available from the Mac App Store.

Linux.