5. 'Trials and Tribble-ations' (DS9)
"Trials and Tribble-ations" is one of the few episodes that shows real, unvarnished joy. In the time travel episode on DS9, Sisko's crew was forced to live through the events of "The Trouble with Tribbles" from The Original Series.
The classic material and the current personalities work together perfectly. The episode looks very light on the surface, but it sneakily celebrates Trek's long life by making canon a pleasant place to play instead of a chore.
The light humor, the sympathetic parody of 1960s styles, and the crew's apparent excitement at seeing Kirk-era luminaries make the show feel like a reunion special with a story involving bombs and tribbles.
In an episode filled with nostalgia, the most emotional, heartwarming moment comes at the end when Sisko dares to meet one of his heroes. Cleverly using footage from the TOS episode "Mirror, Mirror," the DS9 commander comes face to face with Captain James T. Kirk.