I consider Exar Kun as the most powerful Dark Lord in the Star Wars universe, for, without any resources but himself as a mere ghost, it took 14 Jedi to destroy him in "Champions of the Force", where even Palpatine took only two Jedi to destroy him (Luke and Leia in "Dark Empire"). Exar Kun's personal power, plus his hundreds of darkside-focusing pyramids meant that it took ALL the Jedi - thousands upon thousands - to defeat him (in the end of Sith War 40.000 Jedi came to Yavin IV and barely managed to repel Kun's giant energies which he unleashed)! He also is the inventor of famous and vaunted Dual Lightsaber and best swordsman in the history of the galaxy. He started Jedi Holocaust and nearly wiped out all Jedi masters. Not to mention that in the middle of the war with the Republic he came to the Galactic Senate (heart of the enemy!) and freezed with his magic hundreds of Jedi and senators who were present, just to liberate his friend Ulic Qel Droma. Exploits of Exar Kun are legendary, but I admit that Palpatine is also Great One.