Norton is a resource hog and is full of holes.
McAfee uses fewer resources, but it burned me twice.
Nod32 is working great and has protected me thus far.
CA (Computer Associates) is a little slow, but it gets the job done. ZoneAlarm did use CA as the anti-virus in their internet suite.
AVG may be good, but it sucks to try to use it to clean a machine. unless you only have a couple of instances, you will be there for hours clicking, clicking, clicking...
I have heard good things about Kapersky, but I have also heard how it can generate false positives. Several of its updates have caused it to crash or to attack other apps.