um vader isn't as powerful as Exar. in his suite he lost all of his potential yes he does have a good command of the force. when he did have his limbs he had probably the best potential being as the chosen one and all but he never came close to reaching that.
Kun was pretty powerful in the force you know blowing up a star and all which inturn blew up a system. and he was pretty good with a lightsaber especially compared to vader with his bulky suite.