Vader is the best lightsaber fighter. Sidious said Vader powers in the Force as being unparallaled. In Force Awakens Dictionary it is said that through Ren flows bloodline of the most powerful Jedi and Sith (of course implying on Vader). He TKed Sidious which was obvious it wasn't easy for Sidious and that was ROTS Vader. He is the only one with power level besides the Son to cast red force lightning, easily the most powerful force lightning. He did it in subtle form, he probably doesn't use in physical form because of the suit, his force powers are up to that level. The depth of power (which is only latent in Vader case) was always on side of Vader and by a wide margin, no contest. And without reaching up his true depth of power, he already destroys Sidious. At best Sidious could put Vader to the edge which would mean Vader would destroy him. Mortis Anakin is what Anakin would always become in time, revealing him his real nature and his true power level, that was the point of Mortis, not only an amp, showing him what he already had in him and would always become that. The Father obviously knew that. Sidious was always scared of Vader and was shitting his pants of the thought of Vader truly turning on him and going after him.
Vader was always the true terror not Sidious.