I think that Zerg are the most unique:
- They regenerate health over time;
- They use huge flying Overlords as a supply, transport and detection;
- Their buildings are basicly alive non building, but actually the mutated Drone;
- Unlike humans and protoss their strength is in numbers;
- Darwin was right! They evolve;
- They have a town hall in tiers, again they evolve;
- Their buildings regenerate just like all normal units;
- They can infest the terran base to make suicidal humans;
- They can go underground with burrow or underground channel;
- Their units are cheap, cheaper than terran`s or protoss`;
- They only have suicidal units one for ground and one for air;
- They build faster than any other race.
Maybe there is more because i can`t remember it all!
For me Zergs are the most unique.