You have until June, 2018 to test under the ARE 4.0 format. If you think you may transition to ARE 5.0, passing PPP, CDS and SPD will be most beneficial. Here's some strategy.
If you are planning to finish the ARE under the 4.0 format, there is no ideal order. Here are a few facts/tips that may help you decide:
1. Start with a division on which you think you will do well. Success is great motivation to continue.
2. Start with a division you consider to be more difficult. In the event that the division must be retaken you can prepare for other divisions in the meantime.
3. Consult all of the free info from ncarb.org; the ARE 4.0 Guidelines, as well as the Exam Guides and Practice Program (software) for each division. The Exam Guides in particular will give you a good idea of what you'll be facing in each division.
4. If you favor Graphic Vignettes vs. Multiple Choice or vice versa:
The Schematic Design division has no multiple-choice section. It has two graphic vignettes (Interior Layout & Building Layout), each of which have many possible solutions.
The Programming, Planning & Practice division (Site Zoning Vignette) and the Construction Documents & Services division (Building Section Vignette) each have a multiple-choice section. However, the vignettes have only one "correct" answer.
5. Because of the potential for similar multiple choice content, candidates will sometimes take Programming, Planning & Practice and Construction Documents & Services consecutively (in either order).
6. According to NCARB, for 2015 Schematic Design has the highest pass rate at 77% followed by:
Building Systems - 67%
Structural Systems - 66%
Site Planning & Design - 66%
Building Design & Construction Systems - 64%
Programming Planning & Practice - 60%
Construction Documents & Services - 58%