The following is a direct copy-&-paste from my answer to a similar question a few weeks ago but it covers all my thoughts on the matter anyway:
In the fifteen years or so that I've been on AB, this must have been one of the most frequently occurring questions. I'll try to summarise the usual responses:
Firstly, the anti-virus software that's built into all recent versions of Windows seems to be good enough for many people. (It used to be called Windows Defender; it's now Microsoft Defender Antivirus). Millions of people across the world use it perfectly happily without any problems.
However Microsoft Defender does have some critics when you read technical magazines. (Things weren't helped some years when a senior Microsoft executive suggested that it was only a first line of defence). I've also noticed over the years that Microsoft's security programs have a nasty problem of getting 'stuck in a loop' when trying to update themselves, resulting in the computers that they're hosted on running at a crawl. So some people have chosen to look elsewhere for their antivirus protection.
A decade or so ago, AVG seemed to be the most popular freebie used by AB members but it seemed to fall out of favour, possibly because some people found that it was slowing their machines down quite a bit. (Once again, it seemed to be a fault with the update process that was behind the problem). I was one of those who decided to move away from AVG, choosing to use Avast instead (as did many other ABers). I've stuck with Avast ever since and, as far as I can tell from the posts I've read over the past few years, it remains the most popular freebie among AB members.
Kaspersky frequently comes out top of the reviews for full commercial security suites though and, now that they've introduced a free version too, that seems to be doing equally well in the reviews of freebies.
For other possibilities, independently reviewed, see the PC Mag website:
https://uk.pcmag.com/antivirus/120817/the-best-free-antivirus-protection