This may not be the straightforward answer you're looking for, but I believe that the 'easiest' exam is going to be different for each person. It has a lot to do with your previous experience and education as well as what your personal interests are.
If you've been working for a while and have been on site a lot, studying for the Construction Documentation and Administration exam may come more naturally because it will be building on everyday knowledge and experience. If you're still in school or have recently graduated, the Planning and Design exam may feel more similar to school project experience and therefore like the ‘easier’ exam. And finally, studying content that you’re really interested in is just going to come more naturally and will likely lend itself to your success.
A review of the sub-domains and topics for each exam is a good place to start in assessing which exam may be best for you as a starting point. There will likely be one or two that have topics that feel more familiar or less intimidating, or maybe just more interesting to you.
Using the recommended reading list is a must in my opinion. I’ve been able to study each topic on the CLARB issued exam roadmaps using just the recommended reading list and some free online resources and did not use any additional study guides. I did use both the LARE Prep exams and the CLARB practice exams and found them to be useful. I found it important to not get hung up on the score though but to rather use them as a tool to identify gaps in my studying. With the 2023 format change, I found the CLARB exams more helpful than the LARE Prep ones but am hopeful that LARE Prep will update soon.
Good luck!