There isn't much competition.
I would say Andy Webster ex-Hearts and on loan at Dundee Utd from Rangers is very good now that he is fit again and he is probably of a similar standard. Aberdeen's Zander Diamond isn't far from that standard, but he has been injured all season.
Bougherra will be out of Rangers in summer at the latest because they need the money and they are fed up of him returning late from international games.
And, by my logic, Aberdeen's CBs must be better than him because the score over the two Aberdeen v Rangers games this season is 1-0 Aberdeen.