Lord Darkstar
Grandmaster of the AFC
is this the sith who would win in a duel, or the sith with the most force potential?
If it is potential I say Anakin, before he looses any limbs (so I guess AotC before the Dooku duel)
If it is who would win in a duel I say Exar, though this could also go to Revan, Tulak or Marka