Whats the best malware for my new iMac

If Malwarebytes doesn't turn up anything you can run Etrecheck for further analysis.  It is a diagnostic tool that's very useful to us in finding problems. After it runs post the log file here. It will contain no personal information. Allowing full drive access will improve the quality of the report.

To post the log file click on share report and use the Page icon in your reply window.

I would at first try to reboot in Safe mode https://support.apple.com/en-us/HT201262

Be patient: it takes a while to reboot in Safe Mode

None is required. See this article for more information:

Effective defenses against malware and ot… - Apple Community

I think you mean anti-malware 😉

You don’t need any anti-virus apps. If you think you have malware, run free malwarebytes for Mac