Please take the time to look at these websites for detection rate tests:
AV-Comparatives
Both conclude the following if you look at the results:
Best overall is Kaspersky.
Best of the free ones is AOL Active Virus Shield, which uses Kaspersky's antivirus as its base.
Any downsides to using the AOL AVS?
AOL may send spam to the address that you registered the AVS with.
EULA may leave the door open for AOL to implant spyware (although none has been added thus far). To be on the safe side, DO NOT install the optional toolbar that comes with the AVS.
Please read this for more information about the possible dangers.
What is the difference between Kaspersky AV and AOL Active Virus Shield?
The following information about AOL Active Virus Shield comes from delta901 of fatwallet.
All services and processes are identical to Kaspersky
Uses the same antivirus definitions as KAV 6
AOL AVS Activation key does not work with KAV
Web cache scanning feature removed
Proactive Protection removed (this means no monitoring of registry changes, file system changes, or processes monitoring)
Rescue Disk feature removed
Email connection monitoring removed
Trusted Zone settings removed (only added if prompted)
UI changes