I've used the free version of Avast! for over a decade and haven't had any issues at all.

Avast

This might be a little contentious, but the best AV is to not use AV at all. Instead install all untrusted software on a VM with no network device which you can restore to a clean baseline snapshot whenever it gets too infected.

The best free VM is VirtualBox (IMHO)

I'm on a Mac so… well. But all my Windows friends just use Security Essentials and Defender.

Avast Free Antivirus + Hardened Mode

I really like Panda Antivirus, he don't have false positive at all since i used it, don't used very much RAM and CPU usage and is pretty good.

Linux: none.
Mac: none.
Windows: keep system up to date or Windows Defender.

AVAST fast and easy + Malwarebytes,, scans everything including virus, malware, spyware, adware etc.

Win Defender

Avast, because it's the fastest anti-virus I ever used