If we're gonna go there, I find the one built in to Windows 10 is plenty good enough. I could probably try running Malwarebytes occasionally just in case, I suppose.
Bitdefender for me. The simple delight of never having those pop-up ads and notifications that make so many free AV products almost as annoying as what they're supposed to prevent makes it a no-brainer. Really simple, really works.
When I first tried it, I thought I was missing something because the GUI is so simple. It just sits there doing its thing, without fuss. I also put the free version on my phone and was surprised to find it just installed itself and started working. It didn't come with a battery saver, another password vault, or a personal life-choices coaching app. Last few reviews I've seen suggest it's well up with the best as an AV.
I don't find the Avast popups all that annoying, no more than the usual shit encountered when browsing random web sites ("Subscribe to our newsletter" seems particularly ubiquitous these days). I'm only seeing them 2 or 3 times a week, they don't take up much screen real estate, and disappear when ignored. Some of the other annoying stuff like wanting to install browser add-ons are easily skipped, once you know they're a thing (I've probably installed Avast/AVG a dozen times, if not more). So, I'm sure bitdefender is fucking wonderful and so on, but I can't be bothered.
“this is the website that rewards all of the worst features of your personality with a little serotonin hit”
I remember being sat in Geneva airport with my boss and him going “what we need is an IT company based near Middlesbrough” and me taking about 30 minutes to remember that I knew one all along.
Honestly, the relationship has been amazing for us and I still remember where I was when you phoned me as back then we were a relatively new company finding our way.