Reliable virus scan software for iMac

No Anti-Virus software or so-called “cleaning” apps are needed or recommended for Mac OS. They can conflict with Mac's own built-in security. At best they will slow your Mac by using unnecessary resources and at worst will bork your entire system. Your Mac has all the built-in protection you need.

https://www.apple.com/macos/security/

As there are no viruses that affect the mac there is no need for you to install any anti virus app.

If you want to keep on top of malware/ adware then install https://www.malwarebytes.com/mac/

there is a free version.

Macs also do not need any cleaning or optimisation apps.

Read this article,

Effective defenses against malware and ot… - Apple Community

The best virus scan software is simply common sense about software downloads, their originating source (software author only), and the knowledge that adware/malware are the real issues with Macs, and that real viruses (the self replicating kind) are not present on Macs.

Apple has built-in security that fortifies the exclusion of any anti-virus product. For ad/malware, we recommend the free Malwarebytes, which detects and offers to remove any that it finds. That is its sole purpose.

They are all 100% reliable at finding nothing because there is nothing to find.

They are all 100% reliable at causing problems with your Mac.

They are all 100% reliable at reporting false positives, i.e. telling you that you have something that you don't.

As already mentioned, the software between your ears is the best anti-malware.

There are a couple of tools that will remove Adware that you installed--yes, you installed it. It didn't miraculously appear on your Mac. MBAM was already mentioned, but EtreCheck will also find and remove Adware that you installed. You should only ever have to use these tools once because you should teach yourself how not to install Adware: https://discussions.apple.com/docs/DOC-7471

don't pay for anything

don't use something which is "protecting" you all the time that slow your computer down to a halt

if they are from app store then it's less likely they are harmful malware Trojans themselves that can harm your system

I'd stick with etrecheck(free app) once in awhile and a malwarebytes(free version) scan once in awhile