The RSES CMS exams (speciality exams) are the most difficult and therefore the greatest indicator of how much technical knowledge a technician has in the speciality he or she tested in.
The RSES CM exams are the next most difficult and are much more difficult than the NATE exams.
The NATE exams are more practical and easier to obtain. My opinion is that NATE covers the least a journeyman level technician should know. NATE claims that their tests cover what 80% of technicians need to know 80% of the time.
HVAC Excellence has a good series of exams as well. In fact, their exam questions are worded in such a way that they are straight forward and easy to understand. Some NATE questions are not always as easy to understand and some of their possible answers are not always the answer I would consider the most correct.
My book "PAL's HVAC Technician Certification Exam Guide" for sale at the top of the page on this site covers all of this and much more. Plus, it provides many examples of the different types of questions encountered on all the exams no matter which test you decide to take.
There is no one "best" test or certification. They are all different and unique in different ways. Which ones you go with depends upong what you want to achieve, what you will do with the certification, who you work for, what you do or specialize in and what your employer wants you to do.
These questions are also discussed in the book in detail.