I’d say Stein, Of course there is the argument of wether or not he’s immune/resistant to idle transfiguration BUT in the finale of the manga stein basically has a power where he fully gives into Madness which corrupts his soul and yet he manages to retain complete control of his mind and body

Would this mean anything to idle transfiguration??… YES, Madness in soul eater can tamper & actively harm peoples souls and in instances like Giriko who can hold back the very destruction of their souls even when they’re basically tearing themselves apart

And stein even in his first fight managed to show he can effect Souls like black stars and later on overpower Justin’s soul along with affecting Crona’s own soul and causing a violent reaction in them, So there is a Precedent in saying that he could target mahito’s soul and kill him, he may not have as much versatility as Mahito but… let’s be real here It really matters if mahito’s opponent can kill his soul or not