zerg counters
Here are the Zerg Units you should use against the other races:

Against Terran:

Unit Counter
SCV: Zergling, Hydralisk, Mutalisk, Lurker, Anything
Marine: Guardian, Ultralisk, Lurker
Firebat: Guardian, Mutalisk, Ultralisk, Lurker
Ghost: Guardian, Ultralisk, Lurker, Overlord (to spot if cloaked)
Medic: Guardian, Lurker, Any unit
Vulture: Guardian, Mutalisk, Ultralisk
Siege Tank: Upgraded Zerglings, Mutalisks, Guardian, Queen (Spawn Broodling)
Goliath: Upgraded Zerglings, Hydralisk, Ultralisk
Wraith: Hydralisks, Devourers, Scourge, Overlord (to spot if cloaked)
Dropship: Mutalisk, Hydralisk, Devourer
Science Vessel: Hydralisk, Mutalisk, Devourer, Scourge
Battlecruiser: Hydralisks, Devourer, Defiler (Plague)
Valkyrie: Devourer, Hydralisk, Scourge

Against Protoss:

Unit Counter
Probe: Anything
Zealots: Hydralisks, Guardians, Lurkers
Dragoons: Upgraded Zerglings, Guardians (out of dragoon range)
High Templar: Zerglings, Guardian, Queen (Spawn Broodling)
Archon: Guardian, Hydralisks
Dark Templar: Mutalisks, Guardians, Queen (Spawn Broodling). Overload (to spot)
Dark Archon: Anything
Shuttle: Mutalisks, Scourge, Hydralisks
Reavers: Mutalisks, Guardian, Queen (Spawn Broodling)
Observers: Overlords to spot, anything
Scout: Hydralisks, Scourge
Carrier: Scourge, Devourers, Mutalisks, Hydralisks
Arbiter: Devourer, Hydralisk, Mutalisk, Scourge
Corsair: Devourer, Hydralisk, Scourge

Hope this helps in the correct use of the Zerg Race.