Which Free AntiVirus is better?

I suggest Windows Defender if you are using Windows 8 or 10 and Security Essentials for Windows 7. Why? Antivirus software is increasingly becoming targets themselves for attacks. Microsoft follows better security practices. Use Malwarebytes once in a while to scan. Use an adblocker like uBlock Origin and don't download sketchy stuff.
https://www.bleepingcomputer.com/news/security/former-m...

Thanks for your feedback on this. Avast Free 2017 (as well as upgraded versions) allows you to customize your installation and decide which shields and features you want. If you only want to run File Shield for example for a very lean install with less protection, you can do that   If you want to run File Shield with CyberCapture, Behavior Shield, but no Web Shield, you can do that too. You can also customize each component to your liking of course.

You can check out AV-comparative for real world performance results; our team puts system impact at a very high priority!

Avira FREE Security Suite is also a great option.
https://www.avira.com/en/free-security-suite