First forget best, it is subjective.
However, I and many members use the free
Microsoft Security Essentials
Free Malwarebytes and the Windows Firewall
I also use winpatrol. It takes a picuture of your HD. Anything is installed, it asks if you want it. If you say no, it restores what you had. Has other features too. http://www.av-comparatives.org/
This list is already outdated. Microsoft has just finished testing on 2.0. It is now available.