The accident rate in scuba is so low, that unless some agency had a completely HORRIBLE approach, it would be very difficult to use accident rates to evaluate agencies.
What can we use? Well, we can look at the list of skills students are required to learn, which is very similar across agencies. We can look at the amount of required pool time and open water time -- I think most people would agree that more is better. We can look at the process required to become and remain an instructor, with the assumption that, the more dive experience you need to have, and the more hoops you have to jump through to become an instructor, the higher the likelihood that you will at least be a master of your material. (Talent as a teacher may or may not float to the top in such a process.). If you are required to recertify or be reevaluated as an instructor at intervals, then "calibration creep" will be minimized. In addition, if both the factors being evaluated and the criteria for evaluation are public, then the students can participate in the process of ensuring that their evaluation is done as it is supposed to be.
To my knowledge, GUE has the most rigorous process for creating instructors (among other things, you have to be spoken for by an existing instructor as being a reasonable instructor candidate). Instructors must recertify at regular intervals (I think it's 3 years) including passing a fitness test. Descriptions of GUE classes are available on line, with the skills to be evaluated, the factors considered, and a numerical scoring system for that evaluation available for general perusal. GUE has 100% QA, because the student cannot get his card until the QA is done (this may miss a few people who don't pass and don't bother to do the QA for that reason). The GUE OW class requires 40 hours of student contact and a total of 14 dives, although it is not specified how much of that is confined water and how much is open water. That is certainly more than our shop's PADI class, which is one of the longer classes in our area.
However, as I have said before, the class is not widely available, and you do not have to take a GUE OW class to get a reasonable introduction to diving. What really matters is a good instructor with a commitment to training divers well, and enough time in the water to develop a basis of skills.