which antivirus program is used? by most members

What year model is your iMac?

https://support.apple.com/en-us/HT201634

What macOS is it currently running?

https://support.apple.com/en-us/HT201260

High Sierra may have gotten it's last security update back in November, but Mojave and Catalina should continue to receive security updates for some time. see > https://support.apple.com/downloads/macos

My 2¢ worth:

There is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community   

None! see > https://www.apple.com/macos/security/

I been using Mac for over 30 years and never installed any third party Anti-Virus or Cleaning-ware. I do occasionally run the free version of Malwarebytes on my Mac's, but it always comes up clean.

The best antivirus, cleaning app, and overall maintenance app for Mac OS is Mac OS itself. All you need to do with Mac OS to keep it secure and running well is to keep it up-to-date and do periodic (about 1x per week) restarts. Other than that, leave it alone. Adding third party antivirus, cleaning, security and other types of maintenance apps to Mac OS adds no additional level of security. The only thing these apps do is have the opposite affect users want. They make Mac OS slow, unstable, generate odd behavior and make Mac OS appear buggy. 

What I use: macOS - Security - Apple

None. 3rd party anti-virus software on a Mac is worse than worthless.