Definitely First Contact. It has everything a Star Trek film should have, and it's pretty much the whole reason I got into Star Trek in the first place. My favourite Star Trek scene ever is the scene at the end of First Contact where Zephram Cochran and the Vulcan shake hands. Classic.
I also quite like Final Frontier and I think it's very under-rated. The plot stinks, but the interaction between the characters is spot-on, especially when they're singing around the campfire.
Wrath of Khan[/I] is really over-rated and it has easily the worst soundtrack, I think.
Search for Spock is quite good, especially when Kirk breaks down after the death of his son.
The Voyage Home is great, even though a lot of it is just comedy.
Undiscovered Country is really good, but at that point, the crew was just far too old.
Insurrection is just a two-hour TNG episode and a really good one at that. There's no real point to the film, but Riker and Troi get together, not to mention, Riker gets one of the coolest Trek lines ever:
"We're through running from these bastards!" I. Love. That. Line.
Nemesis isn't particularly good, but the battle is probably the best of any of the battles.
Have you even seen any of the Trek movies? I wasn't aware that you were into Trek.
I didn't realize Galaxy Quest was on the list until after I voted. :cry: