The BIG question. In truth none and all of them. Why? Because all agencies have great, indifferent and bad instructors.
It is not the agency that trains divers, it is the professional TEAM. All agencies meet the same ISO standards for diving. However, each agency tries to differentiate itself from the others. So pre-dive safety checks are different but cover the same ground. CESA and ESA, same sort of exercise but slight variations. Depth some are 18 metres for open water as a maximum, others are 20 metres.
Our advice is as follows:-
If going on holiday and completing a course there (referral) don’t assume it will be a PADI resort. Some resorts overseas are all SSI, not PADI.
Check customer reviews on the dive centre you are choosing.
Check the web site and the dive centres TEAM. What qualifications and experience do they have?
If requiring adaptive teaching, be more cautious. Many claim to be experienced adaptive instrctors but have only done a two day course and have limited experience in the field.
Scuba diving has its risks for sure, however, the right dive centre with the right profesisonal TEAM members can help mitigate the risks. Equally, you as the new student have to play your part. Follow the instructions of the TEAM. Make sure you complete and understand the theory. When on site with the TEAM ask questions, explore their knowledge.
Your dive TEAM should make the experience SAFE & FUN.