I voted Khan, because it is without a doubt the winner, by that I mean it was the most victorious. 3 had more grandeur, 4 had more fun , 1 had more wonder, 6 had more closure, & 5 had the most similarity to the show, but 2? That was the one that made it all possible. It was the one that sold the world on Star Trek movies being a worthwhile thing.
I'm not entirely sure TMP did that. It actually disappointed much of the cinema going audience at the time. It failed to capture the Star Wars fervor of the time, & it also seemed rather derivative of 2001, in style, scope & pacing. That's why there was such a vast ovehaul for the next film. When Khan came out, it revitalized the whole thing in a very palpable way, by shifting the focus, & reinvesting everyone in the characters, in a more epic way than had ever been achieved on the show. THAT sucked everybody in.
I'm not sure I agree TMP was the best film of them all either. It did do very many things that were MUCH better than the others, but the one thing it didn't do was convey the personality that TWoK did. It was that personality that drove the rest of the film franchise. That contribution can not be overstated imho.
And it's hard to argue against that, but I can't fault the film for that. What I can do is recognize that such a greatness, that would be mimicked so much since, truly sings its praises better than any other truth. The reality is that when this tv series was carried for a decade after being cancelled, it wasn't just the stories, or sets that kept people invested, it was mostly the characters & their world, & while TMP was a masterful cinema experience, it gave up some of that familiarity to further the narrative. It was a repurposed pilot script for a new show, that very likely would've left behind much of the old style & flavor of the original show (Very much like TNG did)
As a result, I never truly felt like those TMP versions of Kirk, Spock, Bones etc... were fully representative of those characters. They just didn't act like them all that much, more stale or stifled, not as present, because they were not the focus of the film as much as exploring the unknown of space was (Which is a true to form way to interpret Star Trek too, but still fell a little flat, imho) Frankly, it felt like looking at TOS characters crammed into what would become a TNG format. In hindsight I can see that now, but at that time, no one knew exactly why it didn't fit, because the new narrative hadn't played out yet, if you follow me.