For my money, Star Trek VI is by far the best movie the series has produced, ever. Aside from being by far the superior script and directing of all the movies, I'm not sure how you can possibly top the multi-layered Cold War references. Most especially Christopher Plummer as a Klingon channeling Adlai Stevenson yelling to Kirk, "Don't wait for the translation, answer me now!"
You could put that movie on every Sunday on Space, and I'd watch it every Sunday.
Honorable mention: Chekov in Star Trek IV, asking in faux-Russian accent "where are the nuclear wessels" to passersby in Reagan-drenched America. Yes.