I like TOS best when it offered strong morality tales. Yes, it often went over the top, but so many of the films never even bothered to try. (Anyone care to suss out a moral for NEM?) And so TVH and TUC, to me, are most like TOS.
TMP's moral was invented on the soundstage, tacked on to finish the script. WoK makes a good try at creating a message, though. I fear it was lost on me when I first saw it, in high school; in many ways, it's a film for those in mid-life and older.