I vote for TFF. This has always been high on my list of great Star Trek movies. I think it most captures the feel of the original series of any of the movies. Great Kirk-Spock-McCoy stuff, and a great antagonist in the spirit of TOS. Sybok was a zealot who did some bad things but by the end, he realized he was wrong and sacrificed himself in attempt of redemption. This goes right alongside Charlie Evens, the Romulan Commander (BoT), the Horta, who all where multi-faceted characters who's "evil-ness" was questionable. (Unlike Kruge, or the Borg Queen, or the Sona guy (who's name I don't even remember, god, that movie really did suck) or Shinzon, or even to a degree, Kahn himself.) And "God" fits right in with Apollo, and Trelane. If you ask me, ST5 is the most TOS-like of the TOS movies.
I love it.
Also, I don't get the hate for TSFS. That is a great movie (in spite of Kruge). It also has so many great character moments and really expanded the Star Trek universe in interesting ways.
On the other hand, the TNG movies are all terrible. In response to sonak above, Generations was a train-wreck and the handling of the death of Kirk was the least of it's problems. The stuff with Enterprise-B was okay, but clearly written for Spock and McCoy, but shifted to Chekov and Scot since Nimoy and Kelly wouldn't get on board. The stuff with Worf's promotion on the holodeck was fun, even if it didn't quite make sense to the character of Worf (Why would a party for Worf be on a 19th Century sailing ship? Is he into early Earth wet-navy vessels or a big fan of the British Navy? Who of the crew said "Yeah, Worf's gonna love this!"?). Soran's whole plan about getting back to the Nexus is absurd, dispite the fact that ships are destroyed by it, who cares? Everyone on the ships are snatched up by the nexus, so screw the ship! Why wait 80 years to fulfill your greatest fantasy and kill billions of people when you actually can just fly the freakin' ship into it!?!?! And all the stuff with Lursa and Betor and the torture of Geordi just was poorly thought out and ultimately terribly done. Especially since stuff cut from the movie is still referred to (they put an implant that basically gave him heart attacks on command in order to extract information, but while that scene was cut, Soran still has the line "His heart just wasn't in it" which makes no sense without that scene).
First Contact, is the only watchable TNG movie and even then, it doesn't fit well with what came before it. (But I'm actually less of a continuity snob than I sometimes come across as, so no biggie.) But I will argue that the Borg are way scarier as a collective that has ultimately decided (as the ultimate democracy) to go out and do what they do than they are if controlled by one villainous leader than makes them go out and do evil. And most everything that happens on Earth in the 21st Century doesn't make sense if you really think about it, but it's much easier to gloss over the problems in First Contact than it is for Generations. Also, the movie's ending seems to establish that time travel can be preformed much too casually for my taste. At least in ST4 there was a sense of danger and that one false move could send you into the Sun (even if the clock-wise to the future and counter-clockwise to the past was a bit hard to swallow).
I won't even waste my time explaining the failings of Insurrection and Nemesis.
--Alex