Except that Vader wasn't the "most powerful being" at any point. He was "The Chosen One". He had a purpose, he dropped Sid down an energy shaft. Purpose fulfilled, he was no longer needed, thus no longer important in the grand scheme of things. Luke takes over.
In terms of Force Powers and Knowledge, everyone else in that room stands high above Vader. The only thing he might claim over them is raw power. But when his main weakness is lightning, and there's three guys in the room all capable of tearing into him with it, well...Vader's insulation isn't gonna mean jack. It couldn't even stop a jolt from Starkiller, even after Vader improved his defenses against it.
While a number of the Writers may consider these men equal, that is generally because none of them seek to incur George Lucas' wrath. Regardless, when four people in the room are made to fight each other, you take out the weakest one first, and then let two of the others exchange blows and wait for an opportunity to strike when it's most advantageous. All four of them are capable tacticians, but among them Vader is physically the slowest. And while Vader may have adapted to his Cybernetics, it still makes him the most vulnerable amongst the group. It doesn't matter who engages Vader first, or if two of them decide to go at him. It's more than likely they will all seek to eliminate the weaker man first so he doesn't try and shank them while they're busy fighting their supposed equals.
If either Exar Kun or Bane attacks Revan with Force Lightning or another Force Energy based ability, they will wish they hadn't. Revan has already shown himself capable of absorbing that power, channeling and amplifying it into a stronger attack. To which brings the possibility of Kun or Bane striking at Revan with their Force Powers, only for Revan to take that power, add it to his own, and utilize it to quickly take Vader out of the picture. This would of course tell Exar Kun and Bane that using their Force Powers against Revan would not go in their favor. Not unless they could get passed his absorption abilities. That leaves saber combat, and all three of them are prolific fighters.
Bane's Essence Transfer would be a last ditch effort and one he'd only attempt if he was certain there was only himself and one other remaining as a third party could easily jump in and kill the one whose body he is seeking to transfer into, thus killing both Bane and his intended host. It is also unlikely to work on anyone here as each of them possesses great willpower and mental fortitude. So Essence Transfer may end up just killing Bane while temporarily weakening his intended target.
Exar Kun will primarily focus on saber combat and anyone who can get passed his defenses or set him into a trap could deliver a fatal blow. I would actually say that even Vader is capable of doing such, though it would be extremely difficult to strike at Exar Kun with the Force in any meaningful way while defending against a flurry of saber strikes. And, assuming Revan and Bane aren't similarly dueling, it'd leave Vader and Exar Kun both open to assault from a third party, which could potentially be fatal to both of them, leaving the remaining two free to battle for dominance.
And the Chosen One had the potential to be the greatest being ever to exist. Born from the Force itself, Anakin could have been even more powerful than Luke (though not by much). He ended up being 80% of Sidious' power. And Sidious is more powerful than any before him by leaps and bounds. This automatically makes Vader greater than these other combatants.
Vader is not weak to lightning. Not the actual lightning. It is the force behind the lightning that affects Vader. The lightning that Starkiller used against Vader was amplified by Kamino's lightning towers. And Vader simply fell to one knee. Had Vader been weak to lightning, he would have died. Sidious' lightning killed Vader because it is the most powerful lightning ever. This is the lightning that could tear apart fleets of ships. So the lightning of these guys doesn't really help them.
Yes, compared to these guys, Vader is slow. But his raw physical strength could overwhelm any opponent.
I highly doubt Revan could absorb one Kun's force blasts. Blasts that can destroy durasteel walls.
I'm sorry to say, but Revan doesn't have a chance going up against any of these guys. Exar Kun has better saber skills and Force power. Bane destroys Revan with his pure strength and Force power. Vader is the better duelist and has far more power at his command than Revan. Even his tactical skills won't help much here.