Between Jerec and Marr. Hmmm......think I'd have to go with Jerec by a nose.....yeah, real close one though as they are great characters.

Those two are neck and neck.

High inquisitor jerec.

High Inquisitor Jerec overall. Visas is a close second because I like her as a character, but, in the context of the question, Jerec is a far more accomplished dark side master.

Visas is barely a Sith and more of a poor woman who was put in the unfortunate circumstance of being “apprenticed” to a master that is more a Lovecraftian horror than Sith Lord. Said being is concerned only with the satisfaction of his hunger rather than passing on any substantial teachings.