__________________ "Happiness is a lie. Life is horror. The light is always dying all across the universe. The last star will flicker out someday, when it does, all that remains is shadow. And I will be its king!"'-Amahl Farouk
I don't know about Vitiate, to be completely honest, I'm almost 100% Vitiate surpassed him in almost every way. Sadow's only real feat is hitting someone in the head with a brick using the force :/ Kressh, is someone I don't know much about.
Who knows, I'm probably wrong, after all, Ragnos is labeled as one of the scariest Sith for a reason.
First, Vitiate may have surpassed Ragnos at his peak, but when they were both alive, Ragnos ruled unchallenged. Teenaged Vitiate had just consumed his planet, and Ragnos was gracious enough to make him a Sith Lord. This would be like a tiger ripping through your city, and your mayor comes out and lets it have a Pets Mart, because he's amused.
Second, Sadow's expertise in Sith sorcery and the Force is pretty legendary. His Force illusion armies, amped by his Meditation Sphere, nearly destroyed the Old Republic, fighting across multiple fronts/planets and only failed because Gav betrayed his location. He created the amulets which made neophyte Kun an uncontested badass, created the knowledge in Sith alchemy and relic construction which later Sith such as Freedan Nadd, Kun, and even Sidious plundered and respected, and made a ship that could chuck the cores of stars, even in the hands of a relative weakling like Aleema Keto.
Third, Sadow and Kressh were, for all available evidence, near-equals. Sadow was smarter, I felt, but Kressh TK'd a statue nearing 10 feet or more, and while doing so was using an amping amulet very similar to the one Sadow possessed. For context, this amulet amplified the rage and dark side power of Exar Kun exponentially, indicating that high level Force relics were pretty much the way to win in the Ancient Sith Empire.
Ragnos himself had a sceptre that could drain the Force from entire planets, so it's not like he was going to be afraid of his underlings. He's also noted as being incredibly calculating, and either outright beat down his opponents for a hundred years or fooled them into their own demise. The fact that Jaden Korr beat him is, tbh, baffling. But then again Exar Kun's spirit was much weaker than his physical self as well. Who knows?
Bottom line? In terms of most righteous Sith of all time, Ragnos is well well up there with the likes of Barriss, Bandon, and Vindicaa.
He consumed the powerful and mind****ed the rest, iirc. Either eay that's a hell of an achievement for someone who can't legally buy scratch off tickets.