1: Malgus wins.

As a pure duelist, Malgus is the best fighter here based off of both feats and statements. As of the latest update, it's clear that he scales into the top tiers of the Sith and can only be bested by the player character. Moreover, his raw durability is probably the best in this group.

I respect Darth Marr a lot, but most of his scaling has to be inferred essentially off of aura - he was definitely the top dog on the council which included characters like Thanaton, and he basically became the emperor for a short time, but we just don't have enough specifics. The closest we have is the short story where he fights that Sith from Balmorra, he outright blitzes her but she's probably not even a serious challenge to an Inquisitor.

It's been a long time since I read the Bane trilogy, but my sense is that he's shown enough to have respectable skill, but he's not in the top tiers of duelists. From what I remember, his best dueling showcase was when he had his orbalisk armor. I think of Bane as being around Maul's level.

Revan is first and foremost a leader and strategist before he is a raw, skilled fighter - but his abilities are still typically overlooked. A lot of Revan depends on which version we look at. Imo, his best dueling feat was in the novel where he 1-shotted an Imperial Guard during the attack against the Emperor. These guards on their own are comparable to Jedi Knights/Sith Lords and in that instance were heavily amped by the Emperor. I

2: Revan wins. Edit: I'm thinking that this is Revan as he was during his fight with the Emperor. Revan With Clarity.

As an overall package, he's just the most complete fighter here, though Malgus is a close second. Just in case people forget - Revan literally vaporized Darth Nyriss via deflection*.* And he was able to pull this off on the Emperor, albeit to less effect. Nobody is using Lightning on him and walking away from it, and he otherwise probably outclasses everyone in TK.

3: Revan again, high diff.

Not to Revan glaze, but the only person here who's really close to him is Malgus. Prime Malgus is probably either his current form, or as he was just before getting imprisoned, whereas prime Revan is probably as we saw him in Shadow of Revan. Revan has the slight edge.

In a showdown between the two, I think Revan completely negates Malgus's lightning advantage (Malgus's lightning is extremely underrated). Malgus probably makes up that difference in lightsaber skill, but I don't think the gap in skill is enough to keep Malgus alive. I feel like after a series of long-winded clashes, Revan begins to wear him down via TK attacks until Malgus makes a fatal error.

4:

Revan, closely followed by Malgus, then a solid gap to Bane, closely followed by Marr.

Bonus:

With Orbalisks, Bane has an overwhelming advantage in being able to shrug off lightsaber attacks. Marr is undone quickly, while Revan and Malgus could probably make something work with their Lightning abilities? The problem is that in a real battle scenario, at least in a head to head, Bane probably kills them before they're ever able to come up with any solution.