Avast! is what I use for personal use (business has the paid version), in conjunction with Malwarebytes (also free for personal use, paid for business/personal use with more auto-type features).
The built in Windows Defender is actually very good, and many people overlook it because it's Microsoft. It's a very strong program when dealing with malware/spyware, which is what the majority of infections are these days (due to their ease of infection).
Of course, the best anti-virus program out there, is oneself. Don't download things that auto-prompt. Don't click on spam email links. Actually read what's in the installer when you've downloaded legitimate software (a lot of shady stuff is built into the installer). Be mindful of where you're downloading programs from. Essentially, educate oneself, as a great majority of the spyware/malware these days that infect people's computers are do to the user.
