4. "The Trouble with Tribbles"
This is one of those episodes that even non-Trekkies are familiar with, and is it any wonder? It's a comic masterpiece through and through! The crew of the USS Enterprise breaks out so much sass with the Klingons, each other, and Deep Space Station K-7's supervisor that, at one point, it's like watching Friends, but in space.
Additionally, I think NBC and Desilu missed out on a golden opportunity when they didn't spin off Cyrano Jones and the unnamed K-7 bartender and trader, who was portrayed by Guy Raymond. The latter's "I've seen everything" personality is perfectly balanced with Jones' energetic and rather goofball vibe. I've seen this episode dozens of times, but these two never fail to make me laugh! How about an animated series? Star Trek: Space Station K-7!
For those newer fans not as familiar with the entire franchise, the K-7 bartender was later given the name Walt Mathison in the non-canon Star Trek short story called The Last Tribble.