Of paramount importance: what most of us still refer to as “anti-virus” is most of the times a combination of anti-virus, anti-spam, anti-spyware… in brief: anti-malware.
Most suites also offer -depending on vendor and package acquired- a firewall, some kind of identity protection, browsing advisor features, data backup and recovery, parental controls, et cetera.
While some products are better in one or more categories, others are in different areas. However, depending on real needs and type of organization, there are many studies, reviews and comparisons between the many different products, versions, and variants.
Also important is the fact that we need to be cognizant that what worked a few years ago, might not do it as well anymore. The threat landscape changes with the ways and at the pace technology does. Your chosen product could have been top-performer half a decade or just a couple of years ago; better to check its effectiveness today.
A good way to know where our selected suite stands is to check often independent third party organizations that test and qualify these products. For instance, for SOHO and consumer-based products visit http://anti-virus-software-review.toptenreviews.com/small-business-antivirus/ and read articles such as http://techtalk.pcpitstop.com/2010/05/13/the-state-of-pc-security/
For business applications, these two sites offer the best testing and reviews:
Virus Bulletin: http://www.virusbtn.com/vb100/rap-index.xml
AV Comparatives: http://www.av-comparatives.org/images/stories/test/ondret/avc_report24.pdf and