Season finale
Star Trek: The Next Generation: "The Best of Both Worlds" (1990) — In the third season finale, the Borg attack several Federation colonies and when the Enterprise finally catches up with them, they kidnap Captain Picard.  The Enterprise'''s normal weapons have very little effect on the Borg ship, so the crew devises a way to channel a massive amount of energy into the main deflector dish, turning it into a massive weapon.  An Enterprise away team boards the Borg ship to try and rescue Picard, but discover that he has been assimilated — he is now Locutus of Borg.  The episode ends with Locutus informing the Enterprise that they will be assimilated as well.  Riker's only response is "Mr. Worf, fire."  This episode (and its conclusion) are widely regarded by fans to be the best episodes of the entire Star Trek franchise.  This was also one of the first cliffhangers outside of a soap opera.