what is the best antivirus and malware software for imac?

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The best is none! Mac computers do not need any sort of anti-virus, cleaners, VPNs, etc. The only thing they are good at is creating false marketing promises and taking consumers' money. In fact, anti-virus and other cleaners cause performance issues, security issues, and make macOS appear buggy. Your Mac is far worse with these types of Apps installed.

Mac computers have protection built-in, which is the best protection for your Mac. For specifics on macOS Security, see: macOS - Security - Apple and Protecting against malware in macOS - Apple Support. Mac computers also have built-in storage recommendations: Optimize storage space on your Mac - Apple Support. You can even use the built-in macOS Firewall: Block connections to your Mac with a firewall - Apple Support. Other than that, all you should do is restart your Mac approximately once a week and keep macOS up to date: Update macOS on Mac - Apple Support. 

Jack

Mac OS is the best antivirus and malware software for your iMac. Mac OS does not require or benefit in ANY fashion by adding third party AV or malware software. Simply keep Mac OS up-to-date, restart the computer about 1x per week and other leave it alone and just enjoy it!

Adding third party AV apps to Mac OS will have the exact opposite effects you desire, they will:

• Make the computer slow.
• Make the Mac OS appear buggy.
• Will offer NO additional layer of security.

See also:

Effective defenses against malware and ot… - Apple Community