No Jedi ever wins a fair lightsaber battle against a fully trained Sith in any of the movies until Return of the Jedi, where Luke beats Vader. Yes, Obi-wan AND Qui-gon beat Darth Maul, but that's not an even fight.
Yes, Obi-wan beats Anakin on Mustafar, but Anakin was a trained Jedi, not a trained Sith. He had some sith training, but not much.
In A New Hope, Obi-wan outright loses to Vader, choosing to become one with the force rather than slice Vader into pieces.
In The Phantom Menace, when Qui-gon faces Darth Maul one on one, Qui-gon retreats.
In Revenge of the Sith, when Palpatine (Darth Sidious) faces FIVE JEDI MASTERS, he kills four of them before they are even ready to fight him. The fifth, Windu, APPEARS to defeat Palpatine, but Palpatine was feigning weakness to draw Anakin to the dark side. Because as soon as Anakin crosses the point of no return, Palpatine unleashes "UNLIMITED POWER!!"We can also look at Count Dooku verses Anakin and Obi-wan, where he takes them both down. Anakin was young and impetuous, but Dooku easily dispatched him. Obi-Wan was in his prime, and the old man roflstomped him.
THEN Yoda shows up after Dooku has already defeated two Jedi and Dooku is forced to run, but again, we can't call those fair odds.
In Revenge of the Sith, Dooku is killed by Anakin, but only after fighting Anakin and Obi-Wan again... not a fair fight. Plus, the Emperor was sitting there pulling the strings.
In the whole series of films (Until the Disney trilogy, which re-writes the rules extensively), no Jedi ever beats a trained Sith in a fair fight until Luke defeats Vader in Return, and this is arguably because Luke called on the Dark Side of the force to help him.
