TWoK, in part because it's been the best-known Trek film for the longest period; even a casual reader of a review of the most recent JJTrek film would likely come across a reference to Khan. Even before Nemesis (the first of X number of films emulating it - maybe the next one will too?), TWoK had already been the best known, in part because it was the first affordable home-video release of a theatrical movie.
I would avoid TVH primarily because Leonard Rosenman's music doesn't hold a candle to that of Goldsmith, Horner, Eidelman, et al., either on its own terms or as a good fit for the movie. To take one example: The music that begins just as the title card "Music by Leonard Rosenman" appears on screen - all wrong for any imaginable version of Star Trek. As far as I know, Rosenman was a neighbor of Nimoy's, and Nimoy gave him the work as a friend. And indeed Rosenman had done respectable work earlier. But even his great contemporary Henry Mancini's best days were long behind him by the 1980s (both born 1924). Goldsmith (born 1929) was one of the very few of his generation to keep composing memorably for screen.
Wow . . . way harsh.
But, as I'm nearing senility myself, I'm afraid I can no longer compose a memorable retort for this post - unfortunately those days are long behind me.
As far as the original subject, which is quite interesting - I'm leaning toward The Voyage Home; light and entertaining. The Wrath of Khan is great, but might be too heavy, though an earlier poster answered one of my objections nicely when saying Khan and Chekov give sufficient background to the subject. The Undiscovered Country would work well too.