Star Trek II: The Wrath of Khan
Directed by Nicholas Meyer. With William Shatner, Leonard Nimoy, DeForest Kelley, James Doohan. With the assistance of the Enterprise crew, Admiral Kirk must stop an old nemesis, Khan Noonien Singh, from using the life-generating Genesis Device as the ultimate weapon.
Every viewing of this film increases my opinion of it. Even reading all the reviews of it from the time, I still think this is the best that TOS movies were able to give us.