Zerglings, by far. They're the second fastest ground unit with the fastest attack. Cheap and easily massable, they also make the most cost-efficient scouts, are the game's best building demolisher, and are the perfect defiler scapegoats. The zergling's potential is also unmatched in UMS maps, as they are the most commonly used unit in bounds, rpgs, madnesses, and micro maps.
Civilians have been used as pilots, scientists, prisoners, hostages, scapegoats, security guards, refugees, etc