I read the Darth Plagueis novel, and didn’t come away thinking of him as an all time great, nor his master. They certainly don’t match up to people like Vader or Revan.
Plagueis was a true Sith; never in his life had he considered betraying the Sith so Vader shouldn’t be in this conversation. Being a Sith is far more than using the Dark Side; it’s being committed to it until the last sliver of life leaves a Sith’s body. As for Revan he has more feats under his belt and may be a slightly more interesting character but Plagueis is superior in terms of his knowledge of the Dark Side (and other esoteric facets of the Force via his midichlorian manipulation) and he did far more for the Sith Order in terms of ensuring their dominance over the Galaxy. There’s not enough of Tenebrous to really even render judgement on him.
Sorry, I thought you were basing your “all time great” thing on them as characters. In that respect, I found both Plagueis and Tenebrous very mediocre.