Candidates:
* Palpatine (Darth Sidious): This is the most obvious answer. He infiltrated the Jedi Order from within, manipulating them and turning many to the dark side. His treachery ultimately led to the downfall of the Republic and the Jedi Order.
* Count Dooku (Darth Tyranus): A former Jedi Master, Dooku turned to the dark side and became Palpatine's apprentice. He played a key role in orchestrating the Clone Wars and leading the Separatist movement, effectively undermining the Jedi Order.
* Anakin Skywalker (Darth Vader): While ultimately manipulated by Palpatine, Anakin's fall to the dark side was driven by his own fears, anger, and desire for power. His actions, especially the execution of Order 66, were deeply treacherous and contributed to the Jedi's downfall.
* The Jedi Order itself: Some argue that the Jedi Order's own failings contributed to their downfall. Their rigid adherence to tradition, emotional suppression, and inability to see the manipulations of Palpatine ultimately made them vulnerable to his machinations.
Arguments:
* Palpatine's manipulation: Palpatine's deception was masterful. He played on the Jedi's weaknesses, exploited their vulnerabilities, and used their own beliefs against them. His influence was so pervasive that many Jedi became unwittingly complicit in his plans.
* Anakin's fall: While manipulated by Palpatine, Anakin's choice to turn to the dark side was his own. He was a powerful force user, but also driven by fear and ambition. His actions directly led to the downfall of the Jedi.
* The Jedi's flaws: The Jedi Order, despite its noble intentions, had its own weaknesses. Their rigid adherence to tradition, emotional suppression, and lack of adaptability made them blind to Palpatine's manipulations and ultimately led to their downfall.
Conclusion:
The "real" traitor is a complex question. While Palpatine's manipulation was the primary catalyst for the Jedi's downfall, the Jedi Order itself bore some responsibility for its own demise. Ultimately, the Jedi Order's failure to adapt and address its own vulnerabilities, combined with Anakin's fall to the dark side, made their defeat inevitable.