ummm...
Zerg does a very high damage, but very weak in defences.
Terran is a middle class, moderate damages and very satifying defences.
Protoss, well what I know of, they are extremely expensive to build. They can produce powerful energy in damage and defences.
I use Protoss coz they look cool

I also like the other 2 races as well. Siege tanks are very durable. Just drop your tanks behind enemies crystal source. I'm sure they can't feed on anything at all