At the end of Revenge of the Sith we see three Venator-class Star Destroyers. The Emperor, Vader, and Tarkin on on the bridge of one of them viewing the Death Star. Is this ship Tarkin's? What is it's name? In the novel Tarkin we are told that Wilhuff's personal ship is the Carrion Spike and that is not a Star Destroyer...
As far as I know it's never named, not even in the ROTS novelization. In the Tarkin novel, Tarkin does get assigned the ISD Executrix as his "personal ship" toward the end: http://starwars.wikia.com/wiki/Executrix In Rebels, the Sovereign is "Governor Tarkin's Star Destroyer": http://starwars.wikia.com/wiki/Sovereign That said - in Tarkin, he does have a Venator at his disposal, the Electrum: http://starwars.wikia.com/wiki/Electrum_(Star_Destroyer) which raises the possibility that it was the lead ship of that squadron, and that it remained with Tarkin for the five years between ROTS and the Tarkin novel.