Let's see...
Exar Kun and Ulic were dead before Malak's time. Not too far ahead, but still beforehand.
I've seen Darth Maul in action. Pretty acrobatic and impressive. But not worthy of an ancient Sith Lord. I don't recall him exercising the Force very much, or to his advantage, and for those who have played KotoR to completion will recall Malak's casual handling of two Jedi just before the final battle. It's not about direct fighting, even though I'm sure Malak would be a finer, more agile swordsman in the long run, with more experience... it's about power. And Malak has power. He can control the daunting Star Forge, which no one else who tried could besides his superior, Revan.
The rise of Sidious is the shining period for the darkside? Say what?
And Maul was hardly as dangerous as Malak. Let's compare accomplishments:
Maul tracked down and harassed two Jedi.
Malak waged open war with the Republic and decimated the planet of Taris.
Maul managed to kill Qui Gon but fell to thehands of Obi Wan.
Malak defeated all Jedi who came before him except for his superior former master, Revan.
Maul's destiny, despite all his training and reputed badassness, consisted of removing Qui Gon Jinn from the big picture.
Malak's destiny included the Jedi Civil War which meant the destruction of the Jedi Order as it was once known. Malak also controlled the mighty Star Forge and was competent in his own right to remain a Sith lord for some time.
I swear, some of you place way too much emphasis on the movies, especially in a EU forum. Consider the possibility, for a moment, that the movies were made with a limited history and with the possibility that they would stand alone on their own, not breed numerous offshoots.
Now, with that in mind, consider again how impressive other Sith Lords have been. Take a moment, think on it. And reconsider giving Maul more props than he is due.