It is difficult to label one antivirus solution as the best of them all. However, given a certain timeframe and the test results recorded during this time for a number of products, you can clearly conclude which alternatives did best.
AV-Comparatives concluded their annual summary report for 2011 with a very clear-cut decision: Kaspersky takes the trophy. It was not a tough decision, since this was the only product to keep up the same score, highest (Advanced+), across all tests.
The year-long assessment included tests such as on-demand, retrospective (shows how good the offline heuristic detection is), whole-product dynamic protection (tests under real-world conditions), performance (impact on the system) and malware removal.
Other top rated products are Avira, Bitdefender, ESET and F-Secure (product of the year in 2010).
This is the result of AV-Comparatives.org, an independent company specialized in testing antivirus products. Other companies may disagree with the result.