The single worse unit? Its probably the Hydralysk. Hydras die so easily, and just about anything kills them, even irradiated valkeries.
Hydras always clump together, and thats how they die... together. Tanks, scarabs, spider mines, firebats, psy stroms, all mean mass destruction to hordes of hydras.
If any noob wins with hyrdas, its only because he finally managed to build more hydras than the noob he was playing.
Hydras are at their best probaly when sneak attacking a base via, nydus canal, and the base they are attacking is defended by what? Thats right . HYDRAS!!!!!