What is the best free antivirus that could be downloaded?

I have gone through quite a few and the best one i have found to actually remove viruses without having to purchase anything is Superantispyware! I use it and Avast on my system.

I had a bad virtumudo virus on my system and superantispyware was the only program that could fix it and i tried alot. i was running avg at the time with spybot search and destroy and not only did they not stop it from happening they couldn't even see it when i ran them.