Possibly the Ventrue Antediluvian
Himself (or Inconnu)
The oldest texts mention the name "Veddartha" as the sire of Mithras, the fourth generation Prince of London. Later sources also mention Veddartha as sire of Maltheas, a 5th generation Monitor of the Inconnu.
However, according to the accounts of Clanbook: Ventrue, it was believed that by the time of Mithras' Embrace the Ventrue Antediluvian had already been slain outside of the walls of the Second City. So either Veddartha was Ventrue himself – not destroyed by the Brujah Antediluvian or Set, as rumor indicated – or Veddartha was the methuselah responsible for Ventru's diablerie.
However, Transylvania Chronicles II gave some advantage to the theory that Veddartha never was the Ventrue Antediluvian when the book gave his name as Maltheas' sire – Maltheas being a 5th generation vampire Embraced many centuries after Mithras. Time of Thin Blood offers more information about Veddartha, who was mentioned in a letter from the 5th generation Inconnu Gracchus to his dearest Hermia, where he voices the opinion of Veddartha on the subject of the Thin-Blooded.
The letter states that the vampire Veddartha regarded the thin-blooded as nothing more than appetizers for the "Feast of the Ages", when the weaker generations would slake and slow the thirst of the Ancients with little cost to the Inconnu's reserves of strength and wisdom (of which he was a part).
Therefore, a common theory assumes that Mithras was originally a 5th generation who committed diablerie on another Cainite of more powerful blood. Thus his sire, Veddartha, would be a 4th generation who is still active during the Final Nights.