Revised best order for scrum.org tests

I don't think there is a "best order".

Overall, I'd recommend starting with PSM I. Although "Scrum Master" is in the name, it's more of an examination of the rules of Scrum rather than anything specific to the Scrum Master role. Anyone working in an organization using Scrum would do well to understand the material that is part of this exam.

After that, it becomes more role-based.

For Scrum Masters, I'd recommend the PSM II and PSPO I next, probably in that order. The PSM II goes deeper into the Scrum Master role. Product management and the Product Owner role are pivotal to setting the team up for success, so having a deeper understanding of that role would make sense. The SPS would be useful if you're scaling to multiple teams on one product backlog, PSK is useful if you're interested in applying techniques from Kanban. PSU would be useful if you have user experience design incorporated into the Scrum Team, but I've been less than impressed with some of the recommended material for exam preparation. PAL is useful for coaching at the executive levels. PSP, PAL, PSK, and PSU can be taken in any order.

For Product Owners, I'd recommend the PSPO I and PSPO II next to gain depth on the role of the Product Owner and product management in the context of Scrum. PSU may be a good follow-up, but the same caveats apply - I've looked at some of the recommended reading associated with it and it's centered on a few ways of organizing that may not be the best for all organizations.

For Developers, I wouldn't recommend any of the other exams. The PSD exists, but I'd rather invest time and effort learning the tools and technologies used by the development teams. Plus, the PSD only covers the software domain. If you're outside of software, its value is limited.

Understanding concepts and gaining experiences are also more important than taking training courses and exams. Training and exams should be used if they add value. I'd also consider who is paying for the exam - the student or the student's employer - as a factor in deciding if and when to take one. I'd also look at other sources for learning (whether that's reading or training and exams). For example, the SPS centers on Nexus, but Scrum, Inc. offers material and training on Scrum@Scale and there is LeSS material as well - knowing these other frameworks for scaling Scrum can be beneficial to organizations.