That's difficult. Is it one on one or power base vs power base?
Power base vs power base:
Darth Jadus - second in power only to the Emperor
Darth Marr - seems to be the leader of the Dark Council
Darth Vowrawn - seems highly influential
Darth Baras - I think he took over Vengeans powerbase and combined it with his own
Darth Vengean - much military
Darth Decimus - led the attack on Corellia, probably much of his power base
Darth Arho - led the attack on Ilum, probably most of his powerbase
Darth Mortis - no powerbase shown, but more powerful than Thanaton
Darth Ravage - iirc no powerbase shown, but more powerful than Thanaton
Darth Acharon - ally of Darth Decimus
Darth Hydra - ally of Darth Decimus
Darth Nox - newcomer, will gain more power in the years that follow
Darth Thanaton - lost the Kaggath against Nox
In a one on one:
Darth Marr - looks like he is the strongest
Darth Jadus - only an army can stop him
Darth Baras - seems very powerful
Darth Arho - gives the republic characters one hell of a fight
Darth Decimus - seems quite strong
Darth Ravage - thinks he could single-handedly "silence Thanaton"
Darth Mortis - Ravage thinks Mortis could single-handedly "silence Thanaton"
Darth Vengean - thinks of himself as pretty powerful and was killed by two above-average apprentices
Darth Vowrawn - has grown old, he is not in his prime anymore, where I would have placed him close to the top
Darth Nox - with the help of the ghosts, and will become even stronger if he/she isn't killed soon
Darth Thanaton - thinks of himself more powerful than he is
After Baras got power, wasn't he stronger than Jadus?