The best antivirus is the one you use. Many are online without one.
I use Computer Associates (CA) Anti-Virus, and have for three years now. It has saved my bacon many times over.
I like it because it pops up when you hit a page with a virus on it, not when it scans. It does very deep scanning, but it is slow when scanning.
If you're interested, here's a link for a free year trial: