I thought Final Frontier was the worst - obviously written from back to front, irrational choices being made to forward the railroad plot the entire way so Kirk / Spock / McCoy can conclude "since this being is not God, and is lying by calling himself God, there must not be a God."
Undiscovered Country was transparently an effort to place IRL current events in the Trek universe. There might be a background universe where current events fit into the fiction nicely. Trek is not it; there is too much that had to have happened 1965-1980 but did not, to get from here to there. ("To Reign in Hell" describes Khan's childhood on Earth and does a good job with it. "Undiscovered Country" is a sloppy second-rate attempt to imitate what "Reign in Hell" achieved.)
My favorite movie is of course Wrath of Khan. I endorse everything said above.
Don't knock the soundtrack from Voyage Home. Best scene in the movie required the soundtrack's cooperation: Kirk and the Obnoxious Punk dude with his boom box "I hate you / and I berate you", "Excuse me, can you turn that down?" " ... / and I say Screw You!" Spock then gives Obnoxious Punk the Vulcan Pinch, and he _just happens_ to hit the OFF button as he faints.
The original movie is pretty good if you want to sit back and relax and just take in a "thinker" movie. There's not much action because you can't fight a super-brain (V'ger) with disrupter bolts. Kirk has to think his way through the puzzle - and for once, Uhura (instead of Spock) is the one who gives Kirk the 'key' to the right answer.