I'd go for Nemesis. It's a tough decision though, the only one I don't like that much is The Motion Picture, too much like the Original Series for me.
The Wrath of Khan had a good, albeit cliché story.
The Search for Spock, The Voyage Home, and the Final Frontier all had good comedy in them as well as not too bad storylines.
The Undiscovered Country had a more serious storyline, which is a good thing after three not-so-serious ones, though the ending was rather cheesy.
Generations was good, like Ren says, because of the Data jokes, and the effects (which were good for early 90s)
First Contact and Insurrection had more realistic stories, and they were entirely TNG cast. Good effect also.
Nemesis had rather unexpected twists all throughout. Everyone said that a main character died, and when I watched the film, there were several points where several of the main characters could have died, though I didn't expect it to be the one that it was (not saying to avoid spoilers
).