There are several reputable labs which test the effectiveness of major anti-virus programs and security suites to include AV-Comparatives.org, Virus Bulletin Comparative Tests, AV-Test.org, NSS Labs Consumer Anti-Malware Products Group Test Report, Anti-Malware Test Lab, MRG-Effitas, etc.
These kinds of comparative testing results will vary depending on a variety of factors to include but not limited to who conducted the testing, what they were testing for (type of threats, attack vectors, exploits), what versions of anti-virus software was tested, what type of scanning engine was used, and the ability to clean or repair. There are no universally predefined set of standards or criteria for testing which means each test will yield different results. As such, you need to look for detailed information about how the tests were conducted, the procedures used, objectivity and data results. Read Anti-Malware Testing Standards Organization: AMTSO Fundamental Principles of Testing.
Each security vendor uses their own testing/analysis methodology to identify various types of malware so the detection results are not always the same.
Further some of the testing criteria and standards may even be misleading.
...for some unknown reason...the renowned German test lab AV-TEST has quietly (there was no warning) modified its certification process. The changes mean that the certificates produced by the new rules are, to put it mildly, pretty useless for evaluating the merits of different AV products...With AV-TEST’s new certification standards, the onus is on the user to carefully investigate the actual results of each individual test…they may find that a product that blocked 99.9% of attacks has the same “certification” as a product that only blocked 55%.
Comparative testing: A bit of background for the uninitiated
And if you're dealing with zero-day malware it's unlikely the anti-virus testing is going to detect anything. It takes time for new malware to be reported, samples collected, analyzed, and tested by anti-virus/anti-malware researchers before they can add a new threat to database definitions.