Some excellent choices--if traitors can be considered excellent, that is. I'd certainly concur with all the names given so far, and I would add Hitler who led his countrymen to believe he was one thing then turned traitor and involved them in his horrific "Final Solution." He was also a traitor in the sense of not keeping his word to Stalin--a fellow traitorous soul.
What a long list to choose from. The recent Aldrich Ames case in the United States was the first to come into my head because of how long he sold secrets, how many American and allied agents were killed because of him, and the fact that he was a traitor for no other motivation than money.
But I think the prize in terms of infamy would have to be Judas Iscariot, the man who turned Jesus over to the Romans for 30 pieces of silver. The fact that people today use the term "judas" to refer to someone who betrays proves that his infamy has truly stood the test of time.
I am going to say that the greatest traitor in the history of the world was Joseph Stalin. I do not know how you are defining a traitor, but I am using a dictionary definition that says a traitor is
one who betrays another's trust or is false to an obligation or duty
Stalin, of course, did not switch sides away from his country or anything (which is often how we define a traitor) but he certainly betrayed the trust that had been placed in him by the people and he was false to his duty to protect the people he ruled.
I pick Stalin because he (directly or indirectly) caused the deaths of tens of millions of people. Some were killed because he thought they would be dangerous to his political power. Others died as a result of his poor economic decisions. But one way or another, he had a duty to help (or at least protect) these people and instead he caused their deaths.
In the history of Tamil Nadu Ettappan is considered to be the epitome of treachery, so much so the word 'ettappan' is today used to denote any one who betrays his own kith and kin.
Veerapandiya Kattabomman was an 18th century Tamil chieftain who was the first to raise the banner of revolt against the colonial British rulers. Even before the first war of Indian Independence in 1857, way back in the 1790s he revolted against the British by refusing to pay taxes to them. The British tried to arrest him many times but could not succeed because of the tenacity and the 'never say die' attitude of the courageous Veerapandiya Kattabomman. Finally, they managed to corner him and trap him and have him hanged because of the betrayal of his neighboring petty chieftain the Raja of Ettayapuram by name Ettappan.
The name 'Ettappan' in Tamil folklore means a betrayer.