I don't know if it's the best but I use Microsoft Security Essential. It has real time protection and it does a pretty good job at detecting all the nasty viruses that try to install onto my pc. Not to mention it's free. I also use the free version of super anti-sypware and malewarebytes once a month also. There's no need to use a paid AV, when these three programs always get the job done.
I know that the best free antivirus software that you can get is Avast. It highly regarded and has very good reviews from tech websites and the customers themselves. I has served me very well and you can't beat free!
I will stand for MSE which is free from Microsoft. The antivirus is really simple to use and it works really well. I have also noticed that it does not take up many computer resources. This is because you will not even notice the extra process of an antivirus running.
I actively use Avast and I find it to be the best antivirus software out there. I use to have AVG on my old computer, but I have since moved to Avast which is able to protect my computer without slowing it down and using up too many resources. I also love the user-friendly interface of it.
I think that the best anti-virus that I have used so far would be AVG. I have used Mcafee before but it's not free, and I noticed that a free anti-virus like AVG does the job well enough even if it's free, so I never really bothered to try other versions of other anti-virus brands.