Ahh... defilers. Capable of crippling armies of Battlecruisers, Carriers, Hydras, Guardians, Defilers, Tanks, Goliaths, etc. etc. Defilers get my vote. In my opinion, they are one of the most devistating late game units, with their plague (which I spoke of already) and their endless use of darkswarm (with consume) which works with basically any melee unit for zerg. Of course it works with ranged units (hydras) against air, but not so much against ground if you are playing an opponent smart enough to move his men into the swarm as well. Defilers are underrated, as well as under used. Just unburrow, plague/darkswarm, then burrow. Works like a charm.