PAO is the best system. Many people who are fans or users of a 3 digit system may disagree but here is my argument. PAO is more effective then all other methods for numbers and cards. It is
A.Easy to learn
B.Easy to master
C. Has better results than a 3 digit system
Here are the best results with PAO:
Discipline Record Competitor Championship Spoken Numbers (1 second) 432 digits Tsogbadrakh SAIKHANBAYAR GMM Taiwan Open Adult 2015
Binary Digits (5 min) 1688 digits Tenuun TAMIR Mongolia 2020
Historic/Future Dates (5 min) 136 dates Enkhshur NARMANDAKH IMM IGM Mongolia 2020
Speed Numbers (5 min) 572 digits Munkhshur NARMANDAKH IMM IGM WMC 2019 2015
Long Cards (1 Hour) 2141 cards Munkhshur NARMANDAKH IMM IGM WMC 2019
Long Numbers (1 hour) 3818 digits Munkhshur NARMANDAKH IMM IGM WMC 2019
Most of these scores besides 2 or three are world records (based on both IAM and WMSC standards and most are far better than the 2 cards 3 digit records for example in hour numbers the best with 3 digit is 3238 by Alex Mullen)
Some may argue
With a 3 digit system you also have to train hard to get to a high level but you also have to take a lot of time to get used to the systems many images which takes time as well.
Both systems require a commitment (I believe PAO to have less of one) but in the end I believe it is a better system.