Avast free for the last couple of years. I was another long time happy AVG user until they started the over-aggressive upsell nonsense.
I used to be pretty happy to recommend Windows Defender too but from all the recent test results that I have seen, it looks like MS have totally given up trying to keep it competitive in any way.
Also use Malwarebytes when I need a second opinion (usually due to Avast's occasional false positives) and I run Comodo Defense+ (part of their free firewall), which is a great alternative to UAC (as well as being a decent soft firewall) and it is probably the only thing that's actually going to protect me from drive-by malware.