yeah you're right,  sion is not defetead 100%, but he is persuaded to turn back to the light side
can you give me your Y!M ID?

I don't know so much about bane, everything I read about him was that he was the founder of rule of two, nothing more
and about sion you could be right, but traya is wiser than sion, she can lie a lot of people and the secret that she is a dark lord of the sith remain undiscovered until she tells the others that she is darth traya

yeah, it's good
but sion cannot be better than traya,  he didn't had any apprentice, he killed lona vash but this is not enough for a "dark lord of the sith" like him
traya apprenticed revan, nihilus, sion, the exile and is a good dark mentor
I read the Jedi Council blamed her because several of his apprentices were seduced by the call of battle, then fall to the dark side
and maul can't be greater than bane who is the founder of the rule of two

what do you mean powerful? with a lightsaber, with the Force or both of them?

to Jduck13:
1) it's not Darth Ragnos, it's Marka Ragnos
2) Vader it's not the most powerful sith lord, the ancient Sith Lords are the greatest (in my opinion)
3) Obi-Wan is not greater than Mace Windu, Windu is almost incomparable in lightsaber combat
4) Luke is not the most powerful duelist
4) Leia Organa is not greater than Exar Kun
, and more others
check wiki and a few more other web sites and read carefully

c'mon Luke is awful with a lightsaber
in my opinion the best sith and jedi with a lightsaber are:
1) Tulak Hord
2) Mace Windu
3) Vader (in his body not when he was a cyborg)