I agree that Sideous was the most ideal. But we mustn't forget the sith emperor on Dromuund Kaas. Although I can't remember if he completely conquered the galaxy he he wait many millennia to build up his forces. He killed his mother AND father in cold blood. His eyes were as dark as the void, he had gathered all the power fullest sith of his time on one planet and drained all of their power and placed it in himself, being only a child at the time...
I'll mention Krayt's crowning act of treachery- He managed to convince one of his biggest enemies to release a bio-weapon on Coruscant that'd wipe out everyone on the planet in order to wipe out the Sith... except the disease was secretly designed so Sith were immune, so he'd do nothing but kill a trillion people and turn everyone again him, shattering any hope of cooperation against the Sith.
If it wasn't for a betrayal and the killing of said enemy, that'd have been checkmate.
I still think Bane's playing of the Sith against themselves to trap them in the Thought Bomb as well as Sidious' underhanded destruction of the Jedi and countless acts of mass murder and genocide both trump Krayt's swath of terror.
"with no competition", I believe that GL himself states the "competetion" to be the golden age of the Jedi. I find Sidious's rise to the top as definitely the more impressive. In an era featuring Yoda and Mace Windu, amongst others, and with far less force-senstive allies he brought about the near utter destruction of the Jedi. Krayt had a Sith army at his disposal and the less impressive oppostion.
Well, that's what I mean, Sidious made it so by the time he was done, the competition wasn't there any more. He started with competition and was so successful there wasn't any.
I'm not disputing in the slightest that Sidious is king in this category I'm talking about 'ranks lower, but still impressive'. Shattered the Jedi, killed half their number, took over the galaxy, killed the Emperor, if it wasn't for a Knight turning against the Emperor would've killed Coruscant and the entire Empire-in-Exile.
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Also Bane had the cajones to basically stand up in front of the entire Sith following and flip them off. He KNEW Kaan could order the Brotherhood of Darkness to off Bane and yet he still defied them at every turn and openly challenged their ways.
I don't think anyone else in Bane's position would have done the same. Most other Sith Lords would have gone with the flow trying to advance in rank and hopefully one day being recognised. Even Sidious wouldn't dare oppose those above him without some sort of trickery in play. Bane opposed them all solely with his own prowess on his side.
I'm always confused why stubbornness (or arrogance) is brought up when we discuss the merits of Sith. Palpatine being ruthless is one reason his character is interesting, and possibly even one factor in his success as a Sith. However, the same does not hold with Bane. Rather, Bane looks to have a mental condition like Oppositional Defiant Disorder.
Bane had a pretty good idea of the limits of Kaan's power and his needs, though. He knew Kaan still had use for him, and knew that Kaan didn't want to seem afraid of another Sith by ordering others to do his dirty work.
If they ganged up on Bane they would have easily taken him. Which is why Bane basically telling Kaan to F*** off shows him having huge balls since Kaan could have ordered his death
Kaan was arrogant and believed he could still manipulate Bane back into the fold. After he sent Kas'im after him and Kas'im failed he believed what Bane said about realizing the error of his ways. Bane was literally the only one of the brotherhood who didn't succomb to Kaan's Force manipulation in the end, though he let Kaan believe his trick had worked after Bane sent him the info on the Thought Bomb and came back to the planet.
Whatever the case, it was clear that Bane inspired enough fear in the other Sith that they were afraid to confront him in open combat. It's not really all that different than a gang who won't take down one member because they fear him. Looking in from the outside it would be easy to say that ten of them have guns and the other is one man alone but it's a little different knowing that if everyone else doesn't stand with you when you take on the most powerful and ruthless killer in your midst that he can crush you like you're nothing, eh? There's also the fact that Kaan likely feared open combat with Bane because it might inspire other members of the Brotherhood to return to the old ways of preying on one another.