This is a very good question, because I rank TFF highly (above TVH and TUC no less) and I know that is an extremely "unpopular opinion."
I think there are several things at play here for me...I'll try to hit the ones that matter most to me.
1. It reminds me (for better and for worse) of the original TOS series far more than any of the other films. It had the same high-concepts (false Gods, mankind needing their pain, etc), the same plotlines (hijacked Enterprise, perilous journey to a forbidden world, etc), and same humor. It's all there. Even the bad visual fx! Overall, it's the "most Star Trek" out of all the Star Trek films, probably.
2. I love the character stuff. Kirk, Spock and McCoy are the Trek "holy trinity" and this film really focuses on their relationship at this stage of their lives. I think the scenes in the observation lounge (all three) are amongst the best in the series of movies for this reason.
3. This is one of the few Star Trek movies that actually created a sense of wonder and anticipation in me. While the other films are great entertainment, they're really not about exploring the unknown or encountering anything fantastic. It's mostly revenge, time travel tomfoolery, or space politics. This one actually was about discovering something new and, even though that end result was ultimately disappointing, I find that I enjoy the journey enough to give it a pass. The movie stimulated my imagination thoroughly.
4. Unlike you, I find the soundtrack to be fantastic. There are some odd little snippets that don't seem to fit (you point out one), but for the most part, the major pieces are amazing. "The Mountain," "The Barrier," "A Busy Man" etc. are all great...and some of the motifs that carry thoughout the soundtrack are perfect.
5. I love the Sybok character. I think this is actually one of the most interesting, multi-dimensional "guest stars" in all the Trek movies combined. Luckinbill played the role extremely well, and it was a nuanced "shades of gray" antagonist...which I really liked.
6. I thought the cinematography and direction was very good. I think (visual fx aside) this is one of the best looking Trek films. The film had a far more dynamic / modern visual style than the films that preceded it (and most of the films that followed it, barring the Kelvinverse era).
7. Nostalgia admittedly plays a big factor. Summer of 1989 was a very happy time for me in my childhood / early teens. I remember I anticipated this movie more than any other Trek film, and many of my circle of friends were equally as excited. TNG was starting to pick up steam...I was probably at the height of my Trek fandom at this point. Also, I will always remember seeing this movie on opening night with my dad....packed theater and
(yes...believe it or not) the crowd was completely engaged and gave it a standing ovation at the end. I remember being shocked to learn later (this was pre-internet days, so I had no opportunity to read people crapping all over it during its run in theaters) that people really disliked it.
So anyway,
Star Trek V: The Final Frontier will always be a personal favorite of mine. I don't begrudge or challenge anyone with a differing opinion, as I know that my position on this movie is relatively unique (let's face it, it's objectively not a very good movie)....but I do think it is much more enjoyable and worthy than most other people do. I have gotten a lot of joy through the years from re-watching this installment...and I am very thankful for that.