Star Wars: Revenge of the Sith
The first battle of Revenge of the Sith is a rematch of Anakin and Obi-Wan against Count Dooku. Unfortunately for Obi-Wan, someone who still strictly follows the rules and regulations of the Jedi, he was taken out of the fight quickly again. However, for someone who has continued to stray closer to the Dark Side, Anakin defeated Count Dooku, even murdering him in the most “un-Jedi-like fashion” possible, a feat which would never have happened without part of him accessing the power of the Dark Side.
Once Chancellor Palpatine reveals himself to the Jedi as a Sith Lord, he proceeds to kill three Jedi masters with what appears to be complete ease. While Mace Windu does unarm Sidious, the Dark Side prevails again as it finally consumes Anakin fully, leading him to cut off Windu’s arm as Sidious uses Force lightning to murder him, giving the Dark Side another victory over the Light.
In Yoda’s second on-screen duel, this time against Darth Sidious, the long-time Jedi Master who is universally known as one of the most powerful Jedi of all time, loses the duel outright. Whether it be in regard to lightsaber fighting or use of the Force, Sidious is visibly superior to Yoda as he defeats him, even forcing Yoda to go into exile.
The iconic duel of Darth Vader vs. Obi-Wan in Revenge of the Sith may be the one exception to the whole the whole theory of the Dark Side prevailing in each interaction with the Light. Although Vader appears to be pushing back Obi-Wan during the entire battle, his arrogance and overconfidence in his power result in his defeat.
It is worth noting a big reason Obi-Wan defeated Vader in the fashion he did was written into the script to allow Anakin to make his complete transformation into Darth Vader. Regardless of the reasons, the Light Side does in fact unanimously win this fight over the Dark Side.