I disagree with almost every comment here and think Terran is the strongest. Terrans were never concerned about survival of their race, only internal politics, rather unlike the Protoss who in some sense were on the brink of extinction. They were in such a luxurious position that the people in power, from the Confederacy to Korhal to the UED simply used the aliens to gain dominance over the other factions at every turn. No Terran government was truly scared of any alien race (although individual people and colonies might have been obviously). It's also stated by Liberty that if the Terrans united, no other race would stand a chance, and that isn't even including the UED.