Plagues. . . no one in the Star Wars Universe can "own" Ragnos. I agree, He might be defeated by Anakin, but likely not Luke. Possible, but doubtful. Ragnos would tire out Anakin severly even if he did lose. Of course, this is speculation about Ragnos' power, but it is most likely true.
Revan's main power comes from his knowledge yes, but he still must have been very powerful in the Force. Definetely not on Anakin or Luke's level, but powerful nonetheless. And even though his raw power is not on Luke's level, his superior knowledge of Force techniques and hidden powers would almost make up for that disadvantage.
And Exar Kun is powerful in both terms of potential and knowledge. He re-established the Sith, if I am correct, and drove the Republic to its knees. He also, as was said, defeated Jedi could give Yoda trouble, if not an outright fight for his life.
Marka Ragnos attacks Anakin, and Exar Kun and Revan attack Luke. Luke puts up a pretty good fight, but falls in a couple of minutes. Then, the two Sith assist the ailing Ragnos, defeating Anakin. Now, if you replaced Kun with Ulic, or gave the Jedi Yoda or a full power Jacen, they might put a better fight. But as it is, I don't think so.