What is the best free anti virus?

Actually the best thing you can do is learn to not do unsafe stuff on your pc. Right now i dont use any antivirus. Seek some courses on how network safety, if you understand how viruses work you might get some insight.

Avast is perfect. It has removed so much trash from my PC that I didn't know was there xD

Your brain! Most antivirusses bog your PC down, beg you to install bloat and have annoying messages. they always come with these tools that are so much better on their own(ccleaner for example or Pc decrapifier)
windows defender is just fine

i think windows defendet is best to use it is reliable and easy to use

but avast premium is good as well tho.
anyway nice topic m8.