1. Force: Vader's feats suggest that he has the edge in raw power, but Exar's sorcery is potent enough to overwhelm JA Luke even as a weakened spirit (and I still think it's implied that full power Kun > spirit Kun + Kyp), so I'd give him the win.
2. Sabers: Vader prob has Force augmentation, Kun has mobility, skill is difficult to discern as both were prodigies, but honestly Anakin seems more prodigious, having pressed Dooku as a padawan and then surpassed him in like three years, etc.
3. All-out: Exar probably wins with his sorcery, if it was enough to take out Luke that easily, but Vader still has a shot if he counters with his telekinesis and then rushes him. Still, for now I'm sticking with Exar for a slight majority.