Hi
A short answer might have been - none. Joking slightly, because "cleaning" apps are the Bain of Mac users, often causing far more problems than they solve BUT:
OnyX, free, and used properly, is widely recommended on our forums. You can download it from here:
OnyX is a multifunction utility that you can use to verify the startup disk and the structure of its system files; run tasks of cleaning and computer maintenance; configure some hidden parameters of many Apple applications; and more.
www.titanium-software.fr
Make sure you get the version specific to your Mac's Operating System. After installation, Open and use the "Maintenance" tab and use only the defaults.
For possible Malware, DetectX Swift is also recommended:
kicking out badware on macOS since 2014
sqwarq.com
That should probably suffice. Neither of these needs to be used that often.
Ian