The Top two candidates were Kaguya and Hamura. Kaguya was shot out of the window and Hamura's case is wonky. But if we assume that the Hamura spirit that Hinata met was a remanent then "Hamura is RDS" is the top theory. The threads which discuss and support this theory are :-
It's probably Lord Jashin.
We know he's real, otherwise, Hidan would not have the power of immortality.
We know he enjoys the souls of sacrifices, as Hidan loses his blessing when he stops providing them.
So why not grant the user infinite sealing power in exchange for the two souls?