If you mean right now I'd say Spriggs because if Bulaga cannot play, or if he plays and gets injured again, there really isn't anybody else. As inconsistent as Spriggs has been, I think Bell or Murphy off the PS would be worse at RT.
If you mean for next year, assuming they keep playing the way they have in their careers to date, which with these guys is a likelihood, I don't really care much one way or the other. Put a gun to my head and I'd say Spriggs, but he hasn't shown much progress so his upside is probably close to capped.