Oh, don't even get me started.
TMP is wonderful. In many ways, it is imperfect, but it's goals, heart, and soul are so apparent and so Star Trek. It also happens to be the most beautiful looking Trek film of all time. It's gorgeous. TWOK is a better movie in many ways, but TMP is the film in the franchise that was least polluted by gratuitous action and felt most like an episode of the TV series. Even if it is slow at times, it's heavily nostalgic for me, and I find the pacing kind of works in its favor to some degree. I used to watch the special longer version over and over again when I was a kid growing up in the nineties, which I had on VHS (I prefer the Director's Cut nowadays, but miss some of the elements of the longer cut).
I love the odd seventies stylistic choices. I love the incredible, borderline gratuitous amounts of alien costumes. I love the slow pan shots of pretty much everything, which builds both tension and awe. I love the Kirk-Spock-Bones interplay. I love the angry klaxon sounds of the original cut. I love the shots of the Enterprise as a little blip against the awesome size of V'Ger's ship (an effects shot that had to have influenced Interstellar a bit). I love, love, love the opening scene with the Klingons, and the menacing torpedo special effects. I even kind of love the transporter accident scene, which was brutal and necessary to show that Kirk needed to check his hubris. I love that they took the basic concept from "The Changeling" and ran with it, because, in all honesty, I don't think "The Changeling" is a great episode, and I'm happy the idea was expanded upon in TMP.
It does kind of have the feel that it's a movie cobbled together from ideas for a television show that never was, but all in all, it's kind of a breathtaking feat of filmmaking that took a lot of effort by hugely talented people to even exist at all. As a film buff, I consider the movie average, but as a Trek fan, it's easily my favorite Star Trek movie of all time and the one I'm most obsessed with.
(Just read the novelization at your own risk...it's the weirdest thing I've ever read.)