As this is the avast! forum, you're probably going to be recommended avast! here.
You can check out detection rates here:
http://www.av-comparatives.org/Avira has better detection rates, but has a few drawbacks, viz: updates can take 15 minutes, if they ever happen at all, the program is nagware, and there's a bug in the update procedure that leaves a process running at 100% CPU usage.
According to av-comparatives, avast! is 'advanced' and AVG standard in terms of detection.
avast! has more features than AVG (Webshield, P2P scanning etc.)
The choice is yours!