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Tulak for sure, he lived around 5000 years before yoda, in the golden age of the sith. And Kreia says that he was the best dueller of his time, in an age where all the duellers were far better than those even 4000 years ago. Also, much was forgotten in 1000 years from Tulaks time to the time of KotOR, imagine how much more would have been forgotten in 4000 more years. Remember, Tulak lived in a time that the sith could blow up suns, and he was the best of them all in duelling. Kreia says that the best in the time of KotOR would be like children to the ancient sith, Tulak was an ancient sith. He would have had increible knowledge and power. He wastes Yoda, hands down.
I agree. In most polls, popularity wins votes. The only poll I've seen where ability beat popularity was Revan versus Vader, which is 31 to 16, in favor of Revan as I type this.
Considering Vader is the third most popular villain of all time after Hannibal Lecter and Norma Bates, that's an impressive margin.