I've used AVG in the past, and pre-Windows 10 it was good, but then it let a few things through it shouldn't have. I've also tried Avast but it seemed to be a colossal memory hawg with not much better protection than AVG under W10
I've been running Windows Defender with good success. I also use CCleaner free to manage cookies and any sleepers that may sneak in that way. It's a good cookie cleaner as well as registry fixer, and even allows you to scan the start up programs to see where a gremlin may be hiding.
I also run Malware occasionally, but it never really picks up anything, which I guess is good.