I've always been happy with AVG. Version 8.0 has been rock solid ever since I downloaded it. I have run Avast on other computers in the past, and it seems to work pretty good as well. Any time one of my customers lack anti-virus, I always tell them to go to www.filehippo.com and pick up a copy of AVG there. They always have consistently fast downloads.
From what I hear, the absolute best anti-virus is Nod32, but that one isn't free. It however holds the claim of smallest computer footprint, and has the "Leo Laporte" seal of approval.