Best to Worst:
Star Trek: The Motion Picture
Star Trek II: Wrath of Khan
Star Trek VI: Undiscovered Country
Star Trek IV: The Voyage Home
Star Trek: First Contact
Star Trek III: The Search for Spock
Star Trek V: The Final Frontier
Star Trek: Insurrection
Star Trek: Nemesis
Star Trek: Generations
Star Trek 2009 is a different movie franchise really. Doesn't resemble Star Trek too much.
Hmm. After a recent marathon I think I'd move Khan to the top and have The Search for Spock and First Contact above The Voyage Home.
Generatons is the only Trek movie I genuinely dislike though. Not a great film franchise but not a bad one either.