Considering his feat as a starved spirit where he chokes a prep'd and melded amped team of Jedi whom were aided by the spirits of Luke and Vodo, then prime Exar Kun is undoubtedly much more powerful than Darth Vader:
http://www.killermovies.com/forums/showthread.php?s=&postid=15835224#post15835224
Here is what Sidious stated about being a spirit:
Having survived the destruction of the second Death Star as a ghostly manifestation, I can assure you that it is a most wearisome state to be in. It is not enough to retain one’s powers beyond death, for a noncorporeal avatar can only do so much.
- Darth Sidious, Jedi VS Sith: The Essential Guide to the Force
More about spiritual existence:
Emperor Palpatine and the ancient Sith Lords Marka Ragnos and Exar Kun managed to preserve their psyches after their deaths, but it seems their spiritual forms were restricted by certain boundaries and limitations.
- Jedi VS Sith: The Essential Guide to the Force
Originally posted by AncientPower
Considering his feat as a starved spirit where he chokes a prep'd and melded amped team of Jedi
For a few moments. And you still haven't come up with a convincing argument for why we should care. At that point, only Streen had a halfway noteworthy Force feat.
whom were aided by the spirits of Luke and Vodo,
Huh? When Luke came in is around the time that Kun lost.