This isn't going to be the basis of this post, but I personally would put the Exile high on my list.
I believe that his personal experiences gave him an enlightened view of the force posessed by no jedi/sith before or since, and as Kreia said "..became stronger for it". What exactly you mean by weakening severely, and that the force is all he has going for him I'm not really sure. I thought that was more or less true of all jedi/sith to a degree. You're perfectly welcome to explain further though if you want.
Anyway, my list, not really taking into account every last detail of lightsaber form/skill or force form/skill. Just the ones that seem to stand out to me as above the rest.
-Sidious
-Vader
-L. Skywalker(really most in the Skywalker line qualify in my book)
-Revan
-Exile
-Yoda
-Kreia/Darth Traya(Just seems like one of the more notable ones,if only for the power of manipulation)
-etc.etc.etc.
But to be perfectly honest, I'd say any Jedi/Dark Jedi before or during the 100 Year Darkness would trump them all.
Those I believe are the ones that Kreia puts Tulak Hord among, "...in a time when many true masters lived."
I'd take any ONE of those Jedi/Sith over the more modern ones in the list.
For example, I'm almost positive that any of those Sith lords in The Valley of the Dark Lords could take on any given Jedi/Sith in the above list.
I mean, could Yoda, or Sidious, or Revan seriously stand alone against the likes of Marka Ragnos, or a master duelist such as Tulak Hord?
I don't know.
<_<
Mabey I'm wrong, mabey this has all been considered already.
Or mabey Pre/Post-100 Year Darkness is ruled out completely in fairness to the more modern/well known.