Hey guys im new,
my opinion is the Zerg, I know they are sort of based of Warhammer but still the idea is that they are a race of aliens that go around the universe assaulting the inhabitants of planets, absorbing their DNA, mutate or evolve into other creatures to help them in their 'campaign'. they even kicked the Protoss out of Auir, they control their massive numbers through the Overmind which controls the Cerebrates which control the Overlords and finally which control all the other Zerg, the only equivalent so far to the Overmind is Kerrigan.
The Protoss although have Psychic abilities and advanced technology lost to both the Zerg and the Terran. Shame really.
And finally Terran, fighting and killing each other like always instead of the Zerg and Protoss, we/they have been through many wars sadly which probably taught them how to use units effectively and thus survived for so long, They would probably kill each other than the Zerg and Protoss.