Which one is the best free windows antivirus software?

Are you looking it for personal or business use?
For business use the selection is really limited:
http://www.businessnewsdaily.com/6740-free-antivirus-business.html
https://www.raymond.cc/blog/free-antivirus-for-corporate-commercial-and-educational-use/

If you are asking about AV for home use, I would probably install avira.
http://www.avira.com/en/avira-free-antivirus
But I can not say it is the best free AV. You are anyway selecting from a bunch of limited AV solutions (and annoying ads).

Comodo antivirus all the way, sophos and clamav

I find AVG suited to my taste.  If you pick a highly rated antivirus, they all do detection well and it comes down to "I like the way this one looks" or "I like this interface better."

I agree 100% that unwanted software is a problem.  It is tiresome to scrutinize every check box and description to find out if one of those "that looks right" options actually installs unwanted commercial bloatware.

shamigc3 --
You asked for "free".
There is no one antivirus app that is best.  Each has it strong points.  The purveyors of malware are very aggressive.
I use Microsoft Security Essentials as antivirus.  
http://www.microsoft.com/en-us/download/details.aspx?id=5201

And scan perhaps once a week with free MalwareBytes.
http://www.malwarebytes.org/antimalware/

No problems for five years.  But I also practice "safe surfing" and try hard to not access sites that carry malware.

I use Avast.  But I always do a custom install to avoid all the 'features' they want to give me.  There are two of them that can turn your computer, especially an older one, into a slug.  One is the Updates scanner that constantly suggested updates that can't be installed on my computer.  Don't remember what the other was.

I want it for personal use.

At our office we use ESET NOD32, its a great software,  you could also check others
http://www.pcmag.com/article/print/256703

Once you have chosen an anti-virus program you should consider extra protection against
CryptoLocker type malware. This type of RansomWare is becoming very prevalent and is
causing much grief out there.

The free version should suit for most purposes.
http://www.foolishit.com/cryptolocker-prevention/