I've been using
Avira AntiVir for over 10 years now. It hasn't let me down yet. (I will continue to use it until it does!)
The basic version is free, but they also provide a Premium version and a complete Security Suite.
I have recommended this to several people over the years and everyone who's tried it seems to like/trust it. Never heard any negative feedback from anyone.
It compares very well against other big names in the industry.