starcraft units are by far the most powerful. compare a marine wearing those heavy suits and huge weapons with a dwarf with an ancient shotgun.
Diablo and his demons were killed by some guys with axes and some wizards. sure they were powerful but then again, its hard to imagine a barbarian killing a marine. and what would they do against air attacks that come form like 3000 feet above them
also its been mentioned how kerrigans skin was almost indestructible and considering how fast the zerg evolve etc. id go with kerrigan