Yamaoka's power guaging is somewhat spot on. Here's what I believe is to be more accurate.
1. Akira: no doubt where he stands.
2. Azami - The Boss ordered him to kill Yamaoka. He'd chosen someone capable of doing that. However, the Boss underestimated his attractment to Yamaoka.
3. Yamaoka
4. Yuukari might have more field experience than Youko. He operated in China with Azami's driver so foreign environment should mean more assessing and adapting.
5. Youko
Wish there is more info to gauge the Boss. He did raise Akira. He has many faces, 2 of which we know. He is the chiropractor and was a wealthy property owner. What makes Akira so deadly is that he gives no threatening aura and can act like a crybaby. That is what the Boss is doing with those faces.