I use Malwarebytes and Avast, both in free mode.
Malwarebytes mostly because it will warn (and stop) an app doing lots of file locks or deletes. ie - malware encrypting files, followed by a hostage payment offer.
Avast to detect viruses - though it has yet to find any after several years. Mostly it annoys me periodically with paid upgrade offers. Or vaguely sinister warnings about 'security threats' like unemptied trash.