This is rather subjective, but I would have to say that Shinji is. This is because that he used to be a long running captain when he was a captain, and he has quite a powerful Zanpakutō which would help him greatly in battle. His calm demeanor which he has also means that he can keep his cool in the midst of battle unlike other Vizard like Kensei. However there are reasons to suggest that they are all the most powerful.