for me it's
Avira.
the latest update of avira is the best and its free.
My favorite anti virus software are Quick Heal,Avast,Macfee,E-Scan and Norton.
I super Agree that avast is a very user friendly and a very useful anti virus of all time.It detects all malicious programs and even online website firewall with ease.You just can't see that on some other free antivirus program.
There are many anitvirus software. I think the best antivirus software are:
1) Comodo.
2) Avast! Free Antivirus.
3) AVG Free.
4) Microsoft Security Essentials.
5) Bitdefender.
There are many free antivirus are available but all are not best so first of all check result and then after tack decision. Currently I am using Kaspersky's license version. It is more better than any other anti virus software because it is more capable to give protection from hackers and viruses.