Interesting question. Most disappointing necessarily incorporates the expectation level.
I will go backwards, starting with the movies I really liked the most.
Exceeded expectations: TWOK, TVH, TUC, FC.
Nothing terribly original here. These are my favorites by far. Despite an hiccup here (Checkov recognizing Kahn) or there (Picard was off in all the movies).
Exceeded expectations but only because they were really low: ST09, BEY.
I was happy for the reboot. The cast did well. Too much action/silliness. Don't like JJ films. Still ticked about Vulcan/Spock's mom.
I hated, hated, hated, the second Kelvin film, so my expectations for BEY were so low that it surpassed them just by NOT being a friggin' bad remake of a ToS movie. I was out of the loop on Shatner/50th rumors, so that had no effect on my expectations.
Met expectations: TMP, TSFS, INS.
I was pretty young for TMP and did not have any expectations about it. I didn't find it disappointing at all. Nor do I now.
A tad disappointed there was no Kirstie Alley for TSFS, but the movie was fine.
As for Insurrection, I liked it. I have no issue with the movie not being a galactic or federation level threat. I thought the infinity in every moment thing was cool. And at least Picard was more himself again.
Below expectations: TFF, GEN, NEM.
Expectations were high after TWOK, TSFS, & TVH, so TFF was a bit of a letdown. Plus, the "center of the galaxy" thing has not aged well.
Was also hyped for the first NextGem movie, so GEN was disappointing. Weepy Picard & Kirk dying.
I do not hate NEM as much as many do, but it was still disappointing. More so in hindsight as it killed off the TNG movies.
I could easily vote for any of the above 3, but my winner is: STID.
Hey, let's destroy Vulcan and kill Spock's mom to create an alternative timeline so we won't be constrained by canon.....and then make TWOK again, badly. With Kirk & Spock acting stupidly throughout.
Hate it.