[Nogrod]

It can't be Boromir for his deception was short-lived and dubious in the first place as he regretted it and tried to heal the damage he had done. He was indeed effective to the good side in the end.

It won't be Denethor either for me as his lie was not effective enough to lead Gandalf and the defence of Minas Tirith astray.

With Gollum there is the problem of only one part of him being treacherous as Frodo's behaviour managed to bring forth the other one: Sméagol against Déagol - even if the dark one could have been said to talk through the good one at times.

Saruman's lies were also not profound enough to get my vote here as he too was revealed as what he was quite early on and he failed badly in the end and managed only through Wormtongue in relation to the Rohirrim.

That would leave Sauron and Wormtongue for me.

Sauron has his credits: swaying the smiths to forge the rings is a case in point. Fooling Saruman, the greatest of the Maiar in the ME and Denethor the steward of Gondor in a tight place are credits for him as well. There is effectiveness to the largest scale.

But Wormtongue also was effective in ther scale of his reach. He managed to fool Théoden for a long time and would have been succesful if Gandalf had not broken the spell he had wowen. Also he managed to fool Saruman himself from his real feelings and capabilities in the end.

So I would need to choose between the two until you give other arguments for me to change my mind on this.