Star Trek: 25 Greatest Next Generation Episodes
10. Conspiracy
Granted, the first season, at least when compared to the later seasons, wasn't necessarily its prime, but this is definitely an underrated episode.
An early draft of the Borg storyline, the story involves an invasion of the Federation by parasitic bug like creatures. Breathing through a quill on the back of the host's neck, the parasites invade the bodies of high ranking Starfleet officers and weaken the Federation's defenses from within.
When the Enterprise returns to Starfleet Headquarters, Picard finds that three high ranking admirals are infected and have cornered him and Riker, leading to probably one of the goriest shootouts in syndicated television history including an officer's head being blown to bits by Picard and Riker.
With the Writers' Strike of 1988 ongoing (which delayed TNG season 2 for two months), this storyline was never properly wrapped up except in TNG novels and ended with a homing beacon being sent by the mother creature across the galaxy for a larger scale invasion.