I personally prefer Bitdefender, but antivirus is useless. Some of the viruses can't be detected. I un-installed mine months ago.
Avast is the best antivirus, i am using it since 3 years, and really happy with the services provided by it.
AVG has always been my favourite. It doesn't eat all my resources, and it's free. So long as you don't visit torrents the free version is fine.
Many people dont know about the Best anti virus go for Microsoft Security Essential and i bet you wont go for any other after that
For me too Avast. Used to have Avira, but were huge problems with renewing the licence... and Avast is totaly free