Second Sister is the obvious answer for a lot of people I think, and is for me, at least on this poll. If we are including every Inquisitor ever, I'd say Jerec would be interesting to bring back, see how his return would affect things, especially if he regains the power of the Valley of the Jedi.

Th Grand Inquisitor

The Sixth. Tie it in with the Eleventh's death, making it a conspiracy.

Eighth Brother