Doens't make any sense. Vader is said to be the most powerful Sith Lord of all time, who say that? The most powerful being in the Galaxy, Darth Sidious. How can u just say that Exar Kun, Bane or even Revan learned more than the apprentice of the most powerful being in the galaxy?
That's the problem with some EU and with some fans of any character, I like Revan, but you're just being a little far here. Bane learned from a holocran of Darth Revan, but BEFORE that many soldiers talks about he, how can u say that Darth Sidious don't know nothing about him? Datacrons still existis, y'll know?
That being said, Vader hands down win this fight for everything he was and he is in the lore. He was said to be the Chosen One, he was bring him by the Force, he have been the One to make the Great Purge of all time, he was the one to adapt in a hard suit and, well.. you know? He was the first Sith to appear in a screen. XD
Vader > the 3 of them (and I say that again, I really like Revan, he is the one that makes me play this game, but I know something or two about the lore here).
Vader is one of the most powerful sith, not THE that belongs to Sidious. It actually might be Vader > the 3 of them, but lore says otherwise being that Kun was still a force to be reckoned with by the time of Luke's jedi order. Granted he wasn't in a physical body but even so.