best anti virus for iMac desktop

Absolutely none. Microsoft was not involved in the development of macOS and no Microsoft viruses can infect macOS, or gain access to the read-only volume on which the operating system resides.

See Apple Platform Security - Apple Support

and Effective defenses against malware and ot… - Apple Community

If you're trying to prevent malware from getting onto a Mac, avoid downloading software from sources other than the Mac App Store and the developer websites.

If you're trying to remove malware from a Mac, click here and run the tool.

Apple's sandboxing prevents malware from affecting iPads and also stops a scanner from checking anything other than its own documents.

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As noted above, malware is easily dealt with with the Malwarebytes app. Beyond that, no viruses can infect Macs. Installing such antivirus apps will unnecessarily cost you money, seriously slow down your computer and often cause other problems. Do not install any antivirus programs.