bitdefender currently holds the top spot and they do have a free version. avg are up and down as far as catch rate goes. personally i have used avira for the last 4 years in combo with payed versions of malware bytes and emsi soft antimalware.
both combos have given me 100% infection free use and i download a lot of stuff... upwards of 25gigs a night every night. not many av's can stand up to that kind of punishment. then again the weakest point in most antiviruses is sitting at the keyboard.(unless your using microsoft essentials... then you may as well jut remove your fire wall and antivirus for what use it is...
I have used nothing but Microsoft security essentials (100% free download fro Microsoft)on my windows 7 PC right from launch and have never had any issues. I check every now and then with malware bytes and it has found nothing there that was missed.
I used to use Kaspersky paid as I was tired of AVG giving false positives and missing stuff. However MSE does just as good a job in my experience and is free.
I agree that the user is most often the weak point, but can only speak as I find as far as MSE goes and for me (I don't do stupid dodgy searches) its been rock solid.