This has already been asked and answered at least a hundred times on Tom's Hardware. Next time read the forums first.
But the best free one would be Avira or AVG or Avast, take your pick. I prefer Avira.
As for paid, they're not much better than the free ones (if at all), but Kaspersky and NOD32 are both pretty good. If you want an all around anti-bad-computer-stuff program you could go with Microsoft Security Essentials. It is ok. Otherwise make sure to get an anti-malware and a firewall along with your anti-virus. Recommend Malwarebytes Free and PC Tools Firewall Plus, which is also free.
Hope that helps!