As per Filoni, Maul is a Vader-realm threat as of TCW, so that suggests him and Vader would have a good fight. Also, as we saw in Rebels, Maul was pretty equal with Ahsoka, who performed admirably against Vader, and that was a Maul who at best hadn't changed, perhaps declined a bit.
As we saw, Savage not even yet in his prime, was able to drive Dooku back and disarm him, due to sheer strength. It's reasonable to assume that this isn't just a fluke, as Dooku does indeed find strength problematic in a duel, let alone against a hulk like Savage. I believe if Savage squared off against Dooku purely in a duel, the evidence shows Dooku can't handle parrying Savage's strikes for very long, which leaves him few options for victory - he doesn't have the Force here.
If the fight plays out with the MVPs fighting each other (Maul and Vader), and the lesser two squaring off (Savage and Dooku), I reason that Savage can defeat Dooku by disarming/pummelling him while Maul and Vader go back and forth. Once Maul and Savage team up, however, Vader is in serious trouble.
Of course a lot can happen in a duel, and it may not happen as ideally as that: however, given that Vader detests Dooku, meanwhile Maul and Savage have exhibited fantastic teamwork, that can only be another point in the brother's favour.
Even if it was Vader vs Savage, Maul vs Dooku, the brothers are in with a shot: I don't see Vader stomping Savage, given Savage's impressive performances against most everyone else he fought, and Maul's style of fighting is pretty problematic for Dooku (erratic, hyper-aggressive, strength-based etc), though in that scenario Vader and Dooku surely have the better match up.
Given that the above version of Kenobi was a shadow of his ofrmer self and that Dooku kicked ROTS Kenobi, it's fairly obvious Dooku would just blitz Maul here.
Oppress wields impressive Grievous + strength given his avility to drive Dooku back, but this will mena little as Vader has already beat an indisputably more powerful version of Grievous.
One may point to Oppress's supposedly unrivalled force abilities, blasting the likes of Adi Galia, but Vader is 80% of and hence holds "much of the power of" Sidious, a force user who has one-shotted Oppress to the force.
Simply put, Dooku blitzes Maul and Vader one-shots Oppress.
The context of the discussion beforehand is who is worthy to be a RoT Sith, who's got the training and skills, and who doesn't. Filoni explains why Ventress and Savage don't have the chops, then explains Maul does - that he's a super dangerous threat, highly skilled, highly trained, in the same realm as Vader. Idk how you can interpret it as being about something other than power.
Maul is in the Vader realm either way, as per Ahsoka. Filoni just cements it.
Quote: "is a super dangerous threat" [...] "He's really adept" [...] "So he's kind of in the Vader realm"
Look, it's quite simple to see what Filoni is saying here. Maul is in Vader's realm as far as his threat level, purely because of his training from Sidious, which sets him apart from the likes of Ventress and Savage. He's saying Maul is more comparable to Vader than these rogue Sith acolytes. Clearly Maul and Vader aren't equals, but it's impressive praise nonetheless.
It's unclear to me if Rebels Maul is better or worse than TCW, but it doesn't seem like a seismic gap either way, seeing as they both have a form of evidence to show they can stand up to Vader.