BitDefender has to be the overall winner right now. Their antiviral package is competetive to Norton and Mcafee and it uses lesser system resources. It makes no sense if the antivirus takes up more resources than the viruses it has to eliminate. Both Norton and Mcafee cripple ur resources, bitdefender is lighter.
Also look @ Avast. It is the best in terms of antiviral protection and utilizes minimal system resources.