Probably Friend, though chances are the bench utility becomes someone like a Jake Glanville, James Segeyaro type of attacking halfback (or whoever is the best player in that role in 2018, 2019 or whenever that might be). The way Smith is going, retirement might still be some time away and his successor might be the next Cam Smith and not even have played first grade yet.
McCullough is under-rated by everyone except Broncos supporters. A large part of our mid season slump is due to him being missing through injury.
While of course all options are a step down from Smith, you've got to remember that when he retires, in all likelihood Farah and Ennis are retired to, and at the moment there is just as big a gap to Nathan Peats, Mitch Rein or whoever is NSW best option. Sometimes also rep football can be the making of a player.
Funny thing about hookers, the role evolves so much over time. Steve Walters was universally regarded as the best hooker ever. Then he retired and almost immediately Buderus was universally regarded the best hooker ever. Then Smith became universally acknowledged as the best hooker ever. Maybe that's coincidence. If not, whoever Australia's next long term hooker is will be one hell of a player.
Im a massive McCullough fan. Never said a bad word about him. He will replace Smith in the Qld jumper for sure and likely the Aus jumper if NSW don't pick Peats in the next year or two.