Barnetts ground is at the top of the scale in Heavyweight MMA grappling terms. I'd say Werdum probably has a slightly better ground game, but i would put Josh ahead of BJJ Blackbelts like Mir or Nelson as far as MMA goes.
But in the grand scheme of grappling he, like Werdum, is good and near the top of the food chain, but not great. The title of greatness should be reserved for those like Roger and Jacare.
Anyway i see Barnett beating everyone but for Overeem, Cain and Brock. I'd give it 50/50 with Junior depending if he could get the takedown or not. He could easily be a top 5 Heavyweight in the world.