Khan
Spock
Undiscovered
Voyage
Motion Picture
Captain Kirk is Climbing the Mountain Why is He Climbing the Mountain Because He's in Love
Khan is simply the best story, best soundtrack, best villain, and best theme. Search for Spock continues to have very personal stakes in a fantastical setting, which I loved.
Undiscovered Country is great, and it works grandly for a final send-off, but it's too 'fate of the world' action-y, and doesn't have quite enough sci-fi. Still a good allegory.
The Voyage Home is really dated, and the soundtrack sucks, though it's got great performances and is quite funny.
The Motion Picture might, I dunno, be good if you're high?
The Final Frontier is bad.