The absolute best antivirus is any OS that doesn't run Windows. My personal preference is Apple OSX.
However failing that, AVG is the best windows AV out there. Ad Aware and spybot are still two of the top malware/spyware killers.
And if you're trying to prevent an infection to begin with, use Firefox as your browser. Better yet, do all your websurfing in a virtual machine with VMware Server (free) Just set your virtual disk to "non persistent" If your virtual machine catches something, just reboot and dump all of the changes. That's how I do all of my windows based surfing and I've never once gotten a malware/spyware/virus infection. Even when I go to different *ahem* free software *ahem* websites.