Nice sig, that's all I wanted to say.
Sith emperor - Implied to be stupidly powerful and even if you do kill him he just caspers into another body somewhere
Sion - Was only able to be killed when someone convinced him to let himself die
nihlus - Could only be killed due to the protagonists phlebotinum
Umm...I don't we know enough about the contenders limits to say anything of substance here.