My favorite has to be Avast purely for the boot scanning, it has really helped me get rid of brontock a couple of times, not to sure if any of the other free anti-viruses do boot scans now but about 2 years ago Avast was the only one I could find that did it.
Only problem with Avast registering for a free key now is a bit confusing two people who I recommended Avast to said they couldn't find where to register for free home use and ended up buying it but other than than have only had a few spywares slip through.