For me i would have to pick wraith
wraiths own when they are upgraded fully

what you do is mass wraiths and simply fly into the base and go to the nexus, hatchery, or the terran base and stop them there. instantly it will kill all miners then go for the base.

Another way that wraiths are useful is when you yourself is being attack bye carriers, bc's, guardians and devourers. simply b/c of there ability to cloak and the air missles

so my pick would definatly be the wraith