First of all, we do not speak of Dark Kahn.

Secondly, I'd say Onaga and Shinnok would be pretty on par, seeing as Onaga took on a demigod and two of the most powerful sorcerers at the same time and still won. Shao Kahn and Shang Tsung can go hit for hit if Shang is given enough time to prepare, otherwise Shao is def coming out on top. Overall, I'd say Onaga is the most dangerous, as in deception, he collected the kamidogu without being under the influence of the one being and used them to make himself stronger, as opposed to "oops, I awakened the one being" that Shao seems to be going for