Marka Ragnos versus Darth Sion and Darth Nihilus
A very titanic battle.
Sion is virtually indestructible. His skin can recover even from direct lightsaber wounds. Ragnos holds no sway over him, so he will keep coming back, launching himself at Ragnos. Nihilus's nature is that he drains life from every powerful being with no effort. He could literally rip the force out of Ragnos.
Marka Ragnos was THE dark lord of the Sith. He possessed, over all other Sith lords, a terrifying grasp of the dark side.
I say Ragnos goes down. He can't drain anything from Nihilus, and Sion will just keep recovering from the drain.
On the surface of Malachor.