Avast used to consistently be in top 3 (over a year period it is still #1), with a zero-day prevention rate of about 80-95% every day on this site that does a running daily test of zero-day protection of pretty much all the antivirus programs:
http://www.shadowserver.org/wiki/pmwiki.php/AV/VirusDailyStats
But over the last few weeks it has fallen to the 30-50% range in protection and toward bottom of the long list of programs tested.
You can see weekly, monthly, 90, 180 days, 1 year results and see the slide.
Anyone know what was changed in Avast over the last few weeks?