I'm going to go with Star Trek 6, The Undiscovered Country with First Contact and The Voyage Home being the two runner-ups. Insurrection is what I consider the most underrated.
I think The Undiscovered Country has the best plot and the best villain (recently deceased Christopher Plummer did a FANTASTIC job) and Khan + Borg Queen were the other two great villains. The original three films might be on my list, but I honestly haven't seen them more than once or twice* whilst Voyage Home and Undiscovered Country MUST be 20+ times each, and First Contact at least 8 or 9 (probably more).
* I've seen the end of Wrath of Khan many more times than I've seen the actual film.
First Contact was just such a solid film with a solid cast and was one of the two Action Film Star Treks I liked (the other being Star Trek 09).
Most underrated for me was Insurrection, I like that one quite a lot, and it's often said "it's like a glorified Star Trek Episode" and to that I say "Yeah, that's precisely why I like it."
I describe myself as a little dose of toxic masculinity.
