Mace Windu by far. Although both Count Dooku and Yoda bested him in his early years, he became the single most skilled lightsaber duelist in the galaxy once he refined his skills in Vapaad. Mace Windu defeated Darth Sidious in lightsaber combat even quicker than Yoda did.. He was also confident enough in his abilities to confront Count Dooku and Jango Fett without any help from other Jedi what-so-ever.
Both Yoda and Darth Sidious were far more powerful than Mace Windu, but in a lightsaber fight, they'd both lose, they would need to augment their lightsaber skills with destructive force techniques in order to beat Mace Windu, something that Galen Marek learned to do, which made him far more ferocious in combat than any other Jedi/Sith in history.