It was a tough decision.
[Decay Halo] was probably the most compatible with Ryu, but [Swallow Six Senses] was the most immediately useful and impactful to his battle strength. However, there was still [Soul Chain Battlefield] which was an excellent life saving measure no matter how you looked at it.
If someone was forced to battle one on one with Ryu's soul, his current inability to leap through Realms and challenge past his level would be mitigated somewhat. At the very least, someone would have to be at least three Realms or higher to truly threaten his life.
But there was also a glaring weakness of [Soul Chain Battlefield] as well.