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.
Thanaton was made a DC member by killing someone that basically betrayed the sith.
Then he demanded they make him one, they said they would if he told them one of the emperors plans (That bit confuses me...)
Like i said, down to opinion.
Vow is older and less powerful in combat but his knowledge makes up for it and he would be useful in other ways.