Since I personally do not have any need for Anti-Virus software I do not keep track of these applications.
But when I do have to scan someone else's computer. I use F-prot. As it can be run via DOS or from GNU/Linux. To me, this would make it more "effective" for what I need it for.
Most anti-virus software becomes useless if you can not load the OS to run the scanner. Any software that depends (and hopes) you can load the OS before you can scan what may be infected losses a lot of points to me.