For me, it's a toss-up.
So in the context of the time, at the time, going into a movie with expectations: GEN or STFC would get my vote for ending up being most disappointing. GEN was good overall, but I was itching to see what its successor might do, especially with how Q trolled the audience via Picard as to what awaits in ATG... GEN stumbled but still sold that expectation... with that said, STFC was a fairly massive disappointment. Generic faff that was coasting, feeling like a cast reunion party where they forgot about the fans. Even GEN felt more special, and not trying to be a generic action flick bypassing the rules it set up for itself in the process as flagrantly... yeah, STFC should be getting my vote for this.
So in the context of the time, at the time, regardless of any possible expectations - after seeing the movie: NEM gets my vote. All the writers involved and new names in the movie's production seemed like a good idea: Actors who know their characters contributing to the script to help out the other writers, plus some fresh blood to put the thing together... and to top it all off, STXI was already having ideas bandied about despite it all... but by then, had it gotten made, I probably wouldn't have seen it...
So in the context of right here and right now, after rewatching most of the movies again: TFF, since - like TMP - it shares a plot idea, but TMP did it much better in its own ways.
Other ruminations from decades' past:
I remember summer camp where the chatter among the kids was that Kirk would get a DeLorean or TARDIS and be going back in time. Nothing was said about the whales or the overly-long sitcom feel that the movie engendered. "Fish out of water" is a trope I tend to enjoy, which is the main reason this film worked for me at all. It was good, but even back then I preferred TSFS. Comedy's a weird thing at times...
It was after TVH that gave me a sense of trepidation for every upcoming movie and how disappointed I might be as a result. (TFF was leaving me fairly flat, not just because they tried to use "fish out of water" outside any context in which said trope can even begin to work
in, but TUC was so good I saw it thrice - despite some moments that didn't quite work at the time, or moments that have not aged well (Uhura and Chekov once again being reduced to the butt of jokes a la TFF)...) Granted, I also failed to see the logic of kids wandering around school wearing shirts reading the then-current year "1986" in a garish neon color scheme, as if everyone around them was lacking mental coherence they didn't know what the year was and all while wearing sunglasses indoors too... unless society took a collective time travel trip, which might explain a lot...
INS was okay, but Roger Ebert said it best about how it reused a plot point from FC, which was using it as a joke (though I don't recall him saying TSFS used the same "turning against the establishment" trope and, IMFO, TSFS does this trope infinitely better than anything the TNG troupe kept stumbling over...)
But after 4 years, I was hyped for NEM. A movie where, in the theater, which was over half full at the time, the only time people cheered was when the word "Kirk" was uttered. That's bad. A movie chock full of mostly go-nowhere ideas didn't help. Trek was toast by then.
TMP I never saw in the theater. Whether or not I would have liked it, or understood it, or appreciated it... I would do so far more now, right down to Roddenberry's bizarre ideas for uniforms that were right out of some form of kink my ex would doubtlessly partake in for hours on end.
TWOK I wouldn't have been allowed to see in the theater, and it's remarkable it didn't get rated "R". It's also annoying they reuse footage from TMP, even though Kirk's shuttlepod should have gone to torpedo bay 1's pod hatch instead of in the engineering hull's area (they didn't swap "TORP BAY 2" for "TORP BAY 1" either between the time Khan fries that part of the ship, but if that's port side then why was it "2" to begin with? Forget Chekov being recognized by Khan, that port hole is important!!!
TSFS was a blast, and in a day and age of no home video, the recaps from the previous movie were utterly awesome (ditto for TVH in that regard). Granted, nowadays it's either filler or begging the question of how fancy the recordering equipment was to capture all that detail, but this was another era - another time and place, at the cusp of the revolution of movies sold on video for re-viewing, personal libraries, nitpicking every little silly detail... wait, I just did that a paragraph ago... drat... but the nitpicks aside, TSFS bucks the nonsense claim of "only the even-numbered movies were good". TSFS has flaws, but is still more than the sum of its parts. Something the odd-numbered TOS movies can't quite accomplish, with TMP coming close.
ST09 I had low expectations for - it's a reboot and few have won me over. It was made by a guy who later said he hated the show. Which is okay if they hate the show, but at least try and make it
better and embrace it -- not just cut and paste plot points from all the old shows, without any innovation and coast on
superficial stereotyping for the characters. It's amazing that I gave a chance to the sequel...
...STID I had low expectations for thanks to 09's cashing in on hollow, unsatisfying nostalgia (like junk food or my ex in movie form), but Khan wasn't even needed for this movie to be spectacular. Keep the original John Harrison without the "Khan" nostalgiabate stuff jackhammered in, teaming up with Admiral Robocopguy for the double double-cross, and it's still a winner. So it did end up being better than what I was thinking going in.
STB was anything but disappointing, but the Trek movie phenomenon started to fizzle in 1994 and never rebounded.