It all depends on who you ask tbh.
Everyone has their favorites and many of the Dark Council have special abilities that could set them apart.
As well as alliances/allies/rivals if they every had a royal rumble.
For me, Thanaton was the weakest, purely because he was elevated through brown nosing (In regards to giving away the emperors plans) and killing the Emps apprentice. He never really excelled.
The strongest, IMO is Darth Vowrawn though you will find not a lot will agree.
Even though he wasnt the strongest in combat or force, if it came down to it, he was more cunning and was good at building allegiances and would overall win out in a battle of mind of matter.
He was not made a DC member from the storyline, he sits right under the position according to Zash and Moff Pyron tells you he overthrew his superior to become a DC member so it was not completely brown nosing.
Imo, Vowrawn is weak due to age but is extremely knowledgeable in the arcane.
Jadus and Marr are the two I would argue are the strongest but Jadus's reputation may be due to reputation and intimidation on his part and I just get the feeling that Marr runs the show for the most part. He has been a DC member since at least the first Great Galactic War and most likely a bit of time before that too and he seems to show neither withering with age nor fear when challenged.