Since everybody else is doin' it, I'll jump on the wagon:
Wrath of Kahn
Undiscovered Country (these first two are actually a bit of a tie for me)
Search for Spock
The Motion Picture
Voyage Home
Final Frontier
I have always had a soft spot for The Undiscovered Country. It was the first Trek film I was old enough to be excited about going to the theater for. With the previous ST films I saw in the theater, I was more of tag-a-long going to the movies with my dad.