Palpatine, it was Anakins destiny to kill Palpatine, implying Palpatine is too powerful for any mortal swayed from the destiny of the Force to defeat. That's what was up with the poetic prose of the ROTS Novelization.
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
Registered: Jun 2016
Location: The Throne of the Sheevites
Legends sources:
(please log in to view the image) The Complete Star Wars Encyclopedia
(please log in to view the image) The New Essential Chronology
(please log in to view the image) The Official Starships and Vehicles Collection
(please log in to view the image) Jedi Battles
Obi-Wan and Vader duel on Mustafar while Yoda and Palpatine duel on Coruscant. Palpatine is too strong for the aging Yoda, who escapes with Bail Organa.
The Comics Companion
(apologies for the lack of a scan)
Finally, he saw the truth. This truth: that he, the avatar of light, Supreme Master of the Jedi Order, the fiercest, most implacable, most devastatingly powerful foe the darkness had ever known...
just-
didn't-
have it.
Revenge of the Sith
Even the greatest warriors of the Jedi - Mace Windu, Kit Fisto, even Yoda - could not defeat the Sith.
The Last of the Jedi: Underworld
George Lucas also has given his verdict:
(please log in to view the image) (please log in to view the image) Insider #86