I've mentioned Windows Defender a couple of times recently non-disparagingly. I still believe the best protection for your Windows system is discretion. Just don't go to stupid places.
However it still makes me feel better to have some kind of anti-virus tool lurking in the background just in case somebody tricks me.
The AV-TEST Institute runs a couple of anti-virus bake-offs each year. The most recent results are here.
Tom's Guide has a good summary of the AV-TEST comparison. I love their recap.
Microsoft’s [Windows Defender] Protection score was 5.5 out of 6. For a program that was bottom-of-the-barrel just last year - and comes free with Windows - that’s not bad at all.