I voted for Generations. At the time this film came out, I thought it was one of the best in the series. But after the euphoria wore off, I realized that this movie is a complete mess. A collection of cool ideas (blow up the E-D, Kirk and Picard meet, Ent-B) glued together by an incoherent story.
The Nexus is the laziest bit of writing ever produced - "hey, let's create this generic spacial anomaly that just happens to do whatever we need it to do, and we won't even attempt to explain it." Kirk can come out of the Nexus, but nobody else? Are they both still there?
The lighting was horrific. I assume the idea was to hide the cheesiness of the made-for-TV sets on the big screen (which was also the reason they destroyed the E-D), but the idiotic lighting actually drew more attention to the fact that these were TV sets, rather than cover it up. And what was with the mixing and matching of the TNG and DS9 uniforms? Didn't make much sense.
Data as comic relief was the beginning of the end for his character. They turned his emotion chip, and subsequently his whole character, into a joke, a bad one.
And then the greatest sin of all - killing off Kirk. At least the reshot death is a little better than Soran shooting him in the back, but it was still a bit lame. Star Trek's greatest hero dies trying to grab a remote control. When Soran got away from them, Picard and Kirk could have just let the Nexus take them in again, and try again until they got it right, right?
Moreso than INS or NEM, this one feels like a TV episode, with special guest Bill Shatner, slapped together, and thrown up on the big screen.
The fact that Nimoy wanted no part of this, but DOES appear in XI gives me some hope for the new film...