Ooh, another movie question. The Wrath of Khan, no contest. Superior in every way to Star Trek: The Motion Picture.* I have to love a villain who a) quotes Melville, and b) engages in such ferocious scene-chewing that he shows up William Shatner. Plus, I never fail to enjoy the moment Chekov sees the name Botany Bay and realizes what the hell he's stumbled into. I have long since gotten over the impossibility of Chekov and Khan recognizing each other. Who needs continuity when you've got a great scene like that? "This IS Ceti Alpha V!" And those eels. In the ears. That's an image I've never forgotten.
The Voyage Home is a close second. Mainly because I love the humor: Kirk and Spock learn to swear 20th century American style ("Double dumbass on you!" "They are not the hell your whales" ...Really, guys?), Spock puts the neck pinch on a punk who won't turn off his radio on the bus, and Chekov asks the nice police officer to give him directions to the base where the military keeps its "nuclear wessels" (nothing suspicious about that, right?).
*Although I do sort of have fond memories of the crew discovering the identity of "Veeger." And watching it years after my first viewing and going, "Hey, that's Stephen Collins!"