I think my favorite is Star Trek IV: The Voyage Home. It's pure fun, I had Scotty saying "Hello, computer." as my PC's start up sound for about a year after I saw that movie. Also, I might be able to use it to introduce my aero-mech major friend to Trek because he's excited about a movie with the CVN Enterprise in it. Honorable Mention to Star Trek: First Contact for fantastic action and Picard drama.
My least favorite is Star Trek V: The Final Frontier. And I know it's my least favorite because it's the only Trek movie I've only watched once. Honorable Mention to Star Trek The Motion Picture, because even though I love Trek to death, that movie was boring as hell.
However, I'd probably pick at random any episode from Deep Space Nine season 6 if I wanted to sit down and watch some random Trek :169: