For fans of Star Trek: Deep Space Nine, the U.S.S. Defiant is the best ship in Starfleet, yet it failed the mission it was originally created for: to defeat the Borg. The Defiant (NX-74205) debuted at the start of the DS9 season 3 and it was a component the space station-bound Star Trek spinoff desperately needed since having their own starship opened up the type of stories DS9 could tell. There were actually two versions of the Defiant on DS9; a second Defiant-class ship, the U.S.S. Sao Paulo, was renamed "Defiant" after the original was destroyed during the Dominion War.
After the Battle of Wolf 359 in Dominion War, when the starship was destroyed in DS9 season 7. But thanks to the Sao Paulo, rechristened as the Defiant, Sisko was able to win the Dominion War.
And yet, the only time the original U.S.S. Defiant was called upon to battle the Borg — the reason it was built in the first place — it failed. At the start of Star Trek: First , the Borg sent a Cube with the intention of assimilating Earth in the past via time-travel. The Defiant, commanded by Lt. Commander Worf (Michael Dorn), led a Starfleet squadron to fight the Borg and the ship quickly found itself outmatched. The U.S.S. Enterprise-E commanded by Captain Jean-Luc Picard (Patrick Stewart) rescued Worf and beamed him aboard before instructing the fleet in how to destroy the Cube. The Defiant was left derelict in space, "adrift but salvageable," but it was eventually repaired and returned to DS9 along with Worf after the Enterprise crew defeated the Borg and returned from the 21st century.
In the original script for Star Trek: First , the Defiant was destroyed by the Borg Cube. DS9's executive producer Ira Steven Behr was livid over this, especially since he wasn't consulted that a TNG movie was going to blow up DS9's starship. The scene was then changed so that the Defiant survived. Still, the Defiant was done no favors by First ; Trekkers were left wondering why Worf was fighting the Borg without Captain Sisko and the rest of DS9's main crew, and why the Defiant warped all the way to Earth from Deep Space Nine just to lose to the Borg.
The real-world reason, of course, is that Star Trek: First needed an expedient way to bring Worf, who by then had ed DS9 cast, into the movie. But the plot device reeked of contrivance and it did the mighty Defiant no favors. Commander Will Riker (Jonathan Frakes) mocking the Defiant when he called it a "tough little ship" (which echoed what his doppelganger, Thomas Riker, called the Defiant when he stole it in DS9 season 3) rubbed salt in the wound.
There is, however, an in-universe explanation for why the Defiant was defeated by the Borg: the Defiant-class starships were designed to engage Borg vessels as an attack wing. Therefore, the Defiant alone couldn't destroy a Borg Cube, even ed by other Starfleet ships (until Picard showed them the Cube's weak point). Unfortunately, after Star Trek: First , the Borg fell under the purview of Star Trek: Voyager, which meant the Defiant never got a rematch against the Borg. In truth, Star Trek failed the Defiant and Star Trek: Deep Space Nine's "tough little ship" was denied the chance to redeem itself and show it could do the main thing it was built for.