Going solely off of characters who only lack information rather than those who simply have near-conflicting showings in this post....
Personally, Revan is difficult since he has a lot of hype, but most if not all of his comparisons in dueling are against other unquantifiable individuals who's only true comparison is to Revan himself.
Obviously along with Revan is Malak himself.
I also agree Nox is difficult to place as far as sabers go. Obviously they have HIGH force power yet given their medium saber skills is questionable all around for them. We can assume an, at least, proficient level of skill for a Dark Council member but beyond that Nox has one pure saber skill feat to their name and it's not against the most impressive quality opponent as far as their own saber skill could be estimated.
Darth Sion is definitely one I think SHOULD be fairly skilled with a saber, though whether or not he let his abuse of his immortality turn his saber skills to crap is speculatory, as well as the conditions in which he's beaten many of his opponents, like Master Vash.
Kylo Ren as well, seeing as how if you took his injured loss at face-value, and assuming the injury was anything short of completely debilitating, makes him look VERY bad.
Cin Drallig is also odd for me to place, while being the battle-master of the Order it seems like the general impression of him is rather bad. Though that may be only in reference to characters like Anakin?