I only play protoss but I think zerg is the best race.
Between zergling speed, mutas' speed/attack/cost ratio, roach damage/armor/cost ratio, queens (+all of their abilities), infestors, spine crawlers, brood lords, the ease of tech switching and the massive amount of units that can be produced at once from a small number of buildings, I think they are the most powerful race. Protoss and Terran are about equal, I think, after the latest patch. The only way I can conceivably beat zerg ground forces is to force field the army in half. If he goes muta/ling and is good at it, it's pretty much impossible for me. If I defend the back with my units, the zerglings will own the natural expansion. This can be stopped with cannons, but then I won't have enough to make a good enough ground force to deal with mutas. Not to mention that protoss anti-air units are two: Stalker and Phoenix. These are both fail to hydras or roaches which can easily be switched to
Nothing to say about ZvT, but I'm guessing it's not much easier for the Terran, banelings and all.
-Rank 2 platinum