Tulak Hord was never considered the 'most powerful Sith'. He was stated to be the most proficient Lightsaber combatant of the ancients by Kreia in KOTOR 2. It was his Lightsaber skill and technique that was incredible and acknowledged upon.
Best duelist of the Ancient Sith an era that makes KOTOR era combatants look like "Children playing with toys" compared the ancients Sith. Brought down a star ship the size of a Hammerhead Cruiser with the Force, and broke a siege held by an army of Jedi and later with his own army fought an army of Jedi 1000 strong. Later, he drained the energies from all the fallen Jedi. He also allegedly discovered immortality. The Sith Erghast attempting to recreate Hords ability to become immortal instead created Force Walk and we all know how powerful that technique made Darth Nox.
__________________ "Vader's pulse and breathing were machine-regulated, so they could not quicken; but something in his chest became more electric around his meetings with the Emperor; he could not say how. A feeling of fullness, of power, of dark and demon mastery -- of secret lusts, unrestrained passion, wild submission -- all these things were in Vader's heart as he neared his Emperor. These things and more."
__________________ RealistRacism: "Sheevites, much like the Banites, were meant to increase in power with each member. From Lightsnake to Gideon to Azronger, this was supposed to be the case. However, knowledge must've been lost in some kind of Gravid-like incident, as Az turned out to be a mid-tier debater with a sub-par track record, sh!itting all over Tempest's legacy. Sad."