Im surprised so many people rate The Voyage Home so high and TMP so low. They are the complete opposites of Trek, one a topical comedy the other a true Sci-Fi film. That being said here is my list
Because TVH is an enjoyable, feel-good movie that's compulsively watchable and always leaves you smiling, while TMP is a worthy effort that's perhaps not as much fun to rewatch. I respect TMP, but seldom feel compelled to watch it again, whereas I always get sucked into TVH whenever I stumble onto it on TV.
The way I see it, they're both "true" STAR TREK films in the same way that "The Changeling" and "The Trouble with Tribbles" are both Trek.
(To be fair, I actually rewatched TMP the other day, for first time in ages, and it held up better than I remembered. But then I watched TVH and TUC again and, yeah, they're both much more engaging.)