I used Norton (corporate without the bloatware) for a long time until they were very slow to come out with a 64bit version many years ago. I have since been using Avira, which has some of the best overall scores in testing. I always see AVG mentioned a lot as the best free AV, but I don't think it's even close to the best, just the most popular. But my opinion on AVG is not from personal experience with it, just what I've seen from testing sources over the years.
I've tried BitDefender a year ago and it worked well I think, but too many annoyances drove me from it and it didn't perform well on older hardware.
Lately, recent articles have been calling webroot the best out there.
On a side note, Comodo Firewall is nice.