Nik Lentz has a pretty good guard. Not necessarily for submissions, but it's incredibly hard to pass and get anything going against him. It was actually his downfall against someone like Bocek as he didn't have a problem fishing for guillotines and being on his back.
Lentz was the first guy I thought of. Very good at keeping guys weight off of him, and it's hard to hit him with any serious GnP. He also has pretty good guillotines.
Faber has a fantastic guard, one of the best in any division really, not always used to set up subs, but he has never really sustained any GNP damage or given up a dominant position because of it.
We haven't seen it much, but obviously The Chris is the best.
Honorable mention imo to Yoel Romero. Obviously one of the best pure wrestlers in MMA, but he still gets put on his back pretty often, and has been doing well at escaping with a rudimentary but highly effective guard.
More like we have never seen it. Weidman could be just as good as Hendo is on his back. His guard game is a mystery and probably will be for quite some time.