[Storebror]

Best Free version is going to be AVG Free.

Best free AV is going to be your brain.
Followed by other (computer program based) AV tools which tend to change their scores that fast, you just cannot say who'll be best next month.

For instance, look AV comparatives report for 2011 here ("Winner" in the category of free AV was Avast): http://www.av-comparatives.org/images/s ... ry2011.pdf
Another reference is AV-Test.org, see here: http://www.av-test.org/en/home/
When crawling through their tests you'll notice that scores change by month, as do the "leaders" in several categories.

Best regards - Mike