Malak is not only the weakest, IMO, but he's also just the worst overall. Firstly, he wasn't "cunning" when he "killed" Revan. It was a pretty cowardly and non-Sith Lord way of doing it. And he didn't actually kill Revan,
. Secondly, he was pretty freakin dumb. He had no tactical prowess at all. He just had the brutal "fly in with guns blazing" mentality. Thirdly, he was horribly designed. He's got a horrible voice and that metal jaw is just stupid.