In order of how I like to watch them, which ones I find enjoyable.
1. Star Trek II: The Wrath of Khan
-- How scientific advancement affects military technology and uses, Kirk's journey from spiritual death to spiritual life, Spock's peace and eventual death (that is treated as a gift, not a sombre moment). Exploring who Kirk is and how he got that way through his role as a teacher. Just a rich and multi-layered story.
2. Star Trek: Insurrection
-- An interesting moral dilemna, Picard and the Enterprise crew as more than soldiers stopping a madman (or madmen) from destroying the Federation, the Enterprise, or earth. A unique piece of Star Trek.
3. Star Trek: The Motion Picture
-- a clever and throught-provoking idea of a machine that wishes to make the next leap into consciousness juxtaposed with Spock's search for pure logic. Kirk wishing to find fulfillment again, and finding it, and Decker being without it when he loses Ilia and the Enterprise because of Kirk. In the Original Series, the Enterprise crew hummed in every situation. This ship had a command structure that could've ruined the mission. It isn't well-executed but the material is deep enough that I enjoy it.
4. Star Trek VI: The Undiscovered Country
-- This is where I start to have problems. The Klingons would not have a problem attacking Kirk, plain and simple. The basis of the movie is that Kirk is a bigot and learns how not to be. That is not a strong enough movie for me.
5. Star Trek: Nemesis
-- Shinzon is not menacing enough. There is no conflict between who he is and what he could become in Tom Hardy's performance. Other then that, other than more or less of Shinzon, it's a good picture, and underrated.
6. Star Trek III: The Search for Spock
-- This is the anti-Motion Picture. Great in execution, horrible in story and where it took the film franchise.
7. Star Trek IV: The Voyage Home
-- No consequence for Kirk, and he's given a starship. Bugs me every time.
8. Star Trek: Generations
-- A clunky story with b-rate acting.
9. Star Trek V: The Final Frontier
-- A story that has an interesting premise, but fails on its merits.
10. Star Trek: First Contact
-- The anti-Star Trek. Picard is Bruce Willis, Lily is Guinan to Picard, no one needs to do anything but "ready phasers" unless they are telling George Washington that the United States will become the greatest country in the history of the world. Horrible movie. I hate it.