Name Your Favorite Episodes and Why

Despite its age, many people (from multiple generations) still love this show and watch it from time to time. I had this fun idea of people naming what episodes they love the most and would happily watch more than once. Mine are:

"Devil in the Dark" - I loved the story with the Horta. At first it appeared to be like a "monster of the week" kinda story, but changed when you learned more about the creature.

"The Trouble with Tribbles," because it was such a funny story, particularly with Scotty being goaded into a fight by the Klingons and the barfight that ensued, and the Tribbles are just so danged cute.