20. Count Dooku
Count Dooku is one of a few characters on this list who went from being a Jedi to a Sith lord. However, his change wasn’t quite as drastic as Anakin Skywalker’s, the cherub-faced boy who would become Darth Vader; no one flipped sides like that guy flipped sides. Dooku was an apprentice of Darth Sidious, and became known as Darth Tyranus. This was also when he took up his title as the Count of Serenno.
Let’s backtrack briefly and explain his role as a Jedi. He trained as Yoda’s padawan and later trained Qui-Gon Jinn as his apprentice after he became a Jedi Master. He was high in the ranks of the Jedi when he grew wary of the government corruption and went over to the dark side.
Count Dooku’s most influential period was prior to and during the Clone Wars, wars he helped set in motion There’s an entire animated series about the Clone Wars, so Dooku’s influence was great indeed, ugly as it was.
Dooku also trained Asajj Ventress, who had a fairly prominent storyline during this time. But once he was done with her, he began training Savage Opress, but stopped after discovering that Savage was a plant. He then took Jedi Quinlan Vos under his wing, but that was also short-lived. Other than Qui-Gon, Dooku didn’t have much luck training others after he went to the dark side, which is a plus for the Jedi. However, he did wind up capturing both Anakin and Obi-Wan at separate points in time. The guy meant business, always.