Ooh. Tough.
1. Star Trek III: The Search for Spock. This is the only one where I'm sure of its place in the ranking. It's an excellent film. It's so good, it deserves to be an even film. It's so fun seeing the crew rebel like this. Each character gets a separate moment to shine. Christopher Lloyd delivers a delicious performance as Commander Kruge, filled with dark humor. This is the best work he's ever done (except for, you know, Back to the Future!). Spock's resurrection feels earned, not a cheap plot device.
2. Star Trek: Generations. Mostly for nostalgia factor. I have fond memories seeing this movie on opening night on my 12th birthday. For me, destroying the Enterprise-D was an even sadder moment than Captain Kirk's death. Dr. Soran is the best of all the odd film villains.
3. Star Trek V: The Final Frontier. On any given day this film might rank anywhere between 2nd & 6th place. I do want to be nice to it. It's the only movie that truly takes advantage of the Kirk/Spock/McCoy dynamic. Captain Kirk's confrontation with "God" is classic. The problem is that the production values are so shitty all the way around on this film.
4. Star Trek: Insurrection. It feels more like a 2-hour TV episode than a theatrical feature. However, it's always great to see the Next Generation cast on the big screen. F. Murray Abraham does a fantastic job chewing scenery as Ru'afo.
5. Star Trek (2009). I'm still acclimating myself to this new cast & new timeline. However, no matter how acclimated I get, the plot is still too thin. And enough with the friggin' lens flares! There's more lens flares in this movie than there have been in Michael Bay's entire career.
6. Star Trek: The Motion Picture. Sacrilege, I'm sure. I respect the big ideas this film is trying to convey. However, it's boring as hell and the cast is all too stiff. Stephen Collins is good as Will Decker though.