Well, here's the deal. I tested pretty much all of them on a viscious website that automatically installs virii on your computer. Avast! was the only one out of the bunch that caught them and shut the webpage down before it would even load.
AVG I found to be highly overrated. It lets virii on the computer, and then tells you you have a problem. Avast!, on the other hand, stops the virii from ever getting there to begin with.
It has more advanced detection techniques, it protects on more than half a dozen fronts, unlike AVG and thus I strongly urge you to consider Avast! over AVG.