Actually lets all agree that revan will make any sith look like a pushover thus we dont even kno malaks full power.... not takin his side or anything but malak knew he couldnt have the mantle of the sith he even said that on the star forge and why else would revan take malak as an apprentice if he wasnt worthy jack***
He took him as an apprentice because they knew each other and Malak was the most loyal out of the original bunch. He was also the most easily manipulated and allowed himself to be drug down to the dark side with Revan (although I'm sure the seed of darkness was there already), otherwise I would think Revan would bring along Meetra to become a proper apprentice.
And I would disagree about Revan destroying any Sith or Jedi, he was pretty powerful for his time but even then Jedi and Sith only grow more powerful as time goes on.
Darth Malgus is superior to Malak in every way, Malak is a child in comparison.