Lord Sidious - it was stated that he is the most powerful Sith Lord of all time.
For one reason, he has a lot more knowledge at his disposal, since about 4000 years are between them - Kreia has no way to know about those secrets that were uncovered during that time, like Midichlorian manipulation, Life transfer, Force wormhole. Also, he had a Jedi Holocron (Bodo-Baas' if I remember correctly) and I think it's safe to say that he learned quite a bit of the Jedi's knowldge, too.
Two, Sidious often visited Korriban to seek out the Force Ghosts of the ancient ones, so he possibly learned a lot from them in both Force abilities and lightsaber skills.
Last edited by LordSerion; 06-28-2009 at 12:37 PM.