6. Count Dooku/Darth Tyranus
Dooku was trained by Yoda after being taken in by the Jedi Council when he was very young. Once he was a Jedi master himself, he took Qui-Gon Jinn in as his apprentice. After Jinn’s death, he resigned and later was approached by Darth Sidious to switch sides. Since Dooku wasn’t thrilled with the Jedi Council at that point and had no ties at the time, he took Sidious up on his offer.
When Dooku was part of the Jedi Council, he and Yoda put their skills on display for the younglings to see. Both were masterful with a lightsaber. When he became known by the name of Darth Tyranus, he did find himself up against Yoda again, but this time it wasn’t for a friendly demonstration. Keep in mind that his fight with Yoda came after defeating Anakin Skywalker and Obi-Wan Kenobi.
Anyone who can take on Yoda, Anakin, and Obi-Wan deserves a place on this list. It’s not unusual for someone who goes from being a Jedi to being a Sith to wind up gaining even more power once they let the dark side take hold. We see it with Anakin, but he wasn’t the first to do it.