Tricky question this may help if you fancy an av comparison read:
http://www.av-comparatives.org/seiten/comparatives.html
If I had some cash, I would get Kaspersky suite it has one of the best detection rates, due to the signatures, and good heuristic detection, also its trojan detection rates are very good. Mainly because it recieves most of samples from here I believe:
http://www.viruslist.com. It is fairly easy on resources too.
If you want to go free then AVG, Avast, or Anti-vir (free one has no email scanner) are good also. Indeed the pay version of anti-vir is getting very good write-ups at the moment.