My vote since learning about it is Vipre, made by SunbeltSoftware (also the author of Counterspy).
http://www.sunbeltsoftware.com/ Seems to be very low memory usage, updated regularly, fast full deep scans, and no issues as of yet affecting either playability of games, installing applications etc.
I've been running it for about 6 months now and love it. Ended up using it over AVG, Avast, Kaspersky, and a few others I've tried.