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.
This list is already outdated. Microsoft has just finished testing on 2.0. It is now available.