Best Free Antivirus

 Windows Defender

Windows Defender is one of the best because:

1. Because antivirus do deep modifications to a system so need to know exactly how it works and where the quirks are, Windows Defender is written by Microsoft so they know how windows works (I hope)
2. It's malware signatures are mostly up to date
3. It offers ransomware protection together with Microsoft OneDrive
4. It is made and supported until windows goes bankrupt
5. It will be updated together with windows update
6. It's preinstalled

Run scanning every week and update antivirus regularly to get updated new virus definitions.