At one time AVG seemed to be the leader among ABers but some people reported that it was slowing their computers. (The same is true of Windows Defender).
From what I've read on here, it seems that Avast is probably the most popular (free) anti-virus software used by ABers. I've used for many years (having switched from AVG) and I'm perfectly happy with it:
https://www.avast.com/en-gb/index