Annan and Zuculini have the potential to far exceed Song and become the best defensive midfielders on the game, even if they dont look much good right now.
I gave Camacho as slight alternative as hes more technically gifted and more experienced, but even he can flourish into a world class player given enough games. Ive got him at united on a save in 2015 and hes the best box to box midfielder in the world.
Edit: Ive re-read and realised you werent looking for potential, but a star.
If you want a star man look no further than De Rossi, 20 million might be stretching it though.