How powerful is Exar Kun (potential wise)?

How would he match up against

Vader
Nomi Sunrider
Vitiate
Sidious
Dooku
Satele Shan
Galen Marek
Darth Caedus
Jaina Solo
Anakin Solo
Obi Wan Kenobi

i'd put him around 80-85% of RotJ Sidious, so interpret that as you wish.

Exar Kun is unique..nobody on this list can match him.
I agree with xupremexskillz Sidious would match him.
not sure about the percentage...

nah, vitiate, sidious, and caedus match him i think

I'd put him at around Dooku level, or so. He was powerful, but still pretty young, not studying the dark side as long as some others (the artifacts helping make up for that to an extend). Ulric was his equal in a fight when they met.