There a lot of ways to rate an anti virus, you can look at detection rates, the number of false positives, speed, and the bloat factor to just name a few. I also think a very few payware AV's like NOD32 and kaspersky are a hair better than the better freeware alternatives like Avira and Avast. And for what its worth, the final non beta version of Avast 5 just came out a few days ago so its basically unrated yet.
As for me, I have Avast 5 on my wife's computer and I have Avira personal free on mine. Largely because my wife needs the email pre screening that Avira personal lacks.
But both of our computers have stayed virus free for five years or more, but I also attribute that more to a multilayered security system that does not place all eggs in just the AV. Like Schadenfroh notes, following the Mechbgon advice is an excellent way to patch your all your security holes.
But at the end of the day, that old adage applies: There never was a horse that could not be rode and never a rider that never could be thrown. I just want to be sure any hacker has to work his ass off to get me. Usually they go for easier game.