The best way I can say it is, there is no best. You need to find what fits around you, and your needs. I am currently using Kaspersky Antivirus which seems to have got a lot better since the 2010 version. for the years between then and now I used COMODO but recently got sick of it constantly warning me about nothing and deleting files i needed. Kaspersky hasn't interupted me once, its much more lightweight than it used to be also. I would recommend COMODO or Kaspersky. For free COMODO is top of the game and still keeping up with the likes of Norton and other paid programs. Microsoft's security essentials also does a pretty good job but scanning takes an age and it can be a bit slow on definitions. Both of the others I mentioned used a Cloud as well ad definitions and various technologies such as HIPS.