I actually like all of the TOS movies.
1) TMP
This remains the most ambitious Star Trek movie, but it has issues with execution (thanks to well-documented problems in trying to get everything done on time). It has a sense of mystery and creepiness that draws me in whenever I watch it. While the character dynamics initially seem off, by the end of the film, I more or less recognize everyone from TOS, especially Kirk. Remove any other movie from the movie franchise, and I don't think Star Trek as a whole would suffer as much as if you removed TMP. It's unique. I think most fans, even if they don't like TMP, at least have respect for the film.
2) TWOK
I think TV Tropes has mentioned that TWOK is really a deconstruction of TOS, and it works well on film. I think its strengths have been discussed to death.
3) TUC
It's a murder mystery IN SPACE!!! Even though
The Simpsons would joke about "Star Trek XII: So Very, Very Tired" in 1992, TUC actually feels like the most energetic of the TOS films. What dings it are the out-of-character moments of the TOS crew (like Uhura not knowing Klingon... seriously? Or how about the bigoted views on Klingons that the crew have to make an easy parable about the wrongheadedness of prejudice?). Still, it's entertaining and overall a strong entry in the movie franchise.
4) TSFS
This is like the middle child of the TOS movies. It's a solid movie. All I can say is that I just like TMP, TWOK, and TUC more.
5) TFF
There must be an alternate reality where executive meddling didn't force unnecessary humor into the movie and allowed a bigger budget thanks to the success of TVH. Between silly and out-of-character moments to the unnecessarily-hobbled Enterprise, plus terrible special effects (truly, those effects were "special"), it detracts from what could have been a good movie. There are the makings of a good movie in there. It just never really gets a chance to show itself. Luckinbill as Sybok is probably the only nice madman (and convincingly so!) in the entire Star Trek multiverse, though, so there's that.
6) TVH
TVH is a fish-out-of-water comedy that is both funny and cringeworthy. I don't mind watching it if it's on, but I've never purposefully brought it out of my DVD collection to watch. The climax where Kirk has to blow the hatches is difficult to watch and know what's going on thanks to minimal lighting underwater. At least Starfleet somehow managed to hide the
Enterprise-A behind the
Excelsior for a cool reveal, so there's that.