By unique do you guys mean concept wise, or gameplay wise.
Dunno about gameplay, but in concept Protoss are definetely the most unique.
Terran are just humans and there are a bunch of alien races in Si-Fi that are like the Zerg, for one the flood, or the buggers to some degree (From Ender's Game), and in some ways the borg (Though the zerg "assimilate" DNA as opposed to tech)
@VelikoM
The protoss don't speak.... they don't have mouthes....
They communicate telepathically, and that is how they communicate with humans and the zerg too, thus their communication transcends language, and explains why the humans can understand them.