My personal preference happens to be Kaspersky. However, ESET Nod32 usually ranks slightly higher due to Kaspersky being a little more sensitive and returning a "False Positive" occasionaly. As far as Norton / Symantec goes. They test well against known malware. But, they suck at detecting new malware. (less than 35% effective) Check out Virus Bulletins RAP (reactive and proactive) test results.
http://www.virusbtn.com/vb100/rap-index.xmlESET, Kaspersky, Sophos, and Webroot were among the top scorers. As for the "FREE" anti-virus utilities Avira and AVG pretty much tied while scoring high marks (better than both McAffee and Norton/Symantec) and Avast was only a "hair" less effective. Like Joe Friday said "Just the facts"
Also, PLEASE REMEMBER we are discussing Anti-Virus only. Internet security Suites are another thing altogether.