This is kind of a tough decision. If it was for original cast ST films, it's easy: the worst film is clearly TFF.
TNG however, has three candidates: GEN, INS, and NEM.
Like a few people here (as well as Brent Spiner) and I know I'm in the minority, I didn't think NEM was that bad. I thought it was ok. Yes, it had some major problems, mainly an effiminate, bald, dress-wearing, whiny, frustrated virgin who was supposed to be a younger, evil version of Captain Picard. Shinzon just didn't work for me at all. They wanted him to be on par with Khan and I think they failed miserably. B-4 was pretty annoying too. The movie was awfully slow in the beginning too, and I thought the dune buggy sequence was laughable. On the other hand, the part where Data and Picard escape from Shinzon's ship and the final space battle were great and Data's death was pretty well done. Also, unlike GEN and INS, I felt like they did try to make NEM look and feel like a real epic motion picture, not just a movie for the fanbase.
I really don't think NEM deserved it's horrible rating of 38% on rottentomatoes and it's abysmmal box office. It definitely wasn't one of the great/good ST films (TWOK, TSFS, TVH, TUC, and FC) but it wasn't that bad. Rick Berman, who still can't figure out why NEM failed, had one thing right: all the fans didn't automatically show up for the first weekend (or the subsequent weekend) to see NEM. He blamed that on franchise fatigue, i.e. too much ST. I think it goes beyond that. I think there was too much bad ST or just too much of the same thing with VOY and ENT, which looked to me like TNG v2 and v3. I also think that INS, whose box office was significantly lower than FC, GEN, and TUC, hurt NEM. The fans stayed home the first weekend NEM was out and, when the movie didn't get good reviews or good word of mouth, they waited for the DVD.
GEN suffered from a very bad script (Ron Moore has admitted that), a TV director and a TV score. It has some things going for it: the death of the Ent-D, while lacking the poignance and impact of the death of the original Enterprise, was spectacular. On the other hand, it was destroyed in an incredibly contrived way. And, while the movie starts of great with Kirk's first death and ends with some wonderful scenes of him teaming up with Picard, it's marred by a very muddled and screwed up death for him.
GEN isn't a good movie, but I give it points because it had some big things happen in it and, in that sense it tried. It's sad though, because it really was a missed opportunity. The concept of Picard meeting Kirk and giving Kirk an end, was a great one. The screenplay just didn't do it well at all.
But, despite these flaws in NEM and GEN, I think INS is the worst. The poster who pointed out the dubious morality of the film, something Brent Spiner criticized as well (he said the Ba'ku weren't worth saving because they wanted to keep their fountain of youth to themselves) was right imo. I also thought the villain, Ru'afo was pathetic, a whiny plastic surgery victim. The humor was stupid too. I hated seeing Picard singing with Data. Data can sing, yes, but not Picard. Worf's zit and Data's boob jokes were just stupid. Why is Picard always the one on the surface with the gun in the movies, leaving Riker in command of the Enterprise (especially when it got destroyed in GEN?). Yeah, I know it's because Picard and Data are the stars of the movie (according to Rick Berman), but they never gave a logical reason in GEN or INS for sticking Picard on the surface and leaving his ship. Picard is not Kirk, that was the whole point of making him different in TNG. INS was boring too. Frakes, the director even said that. He didn't like the script at all. But, to sum it all up, I thought the biggest problem with INS is the same as the one for TFF (in addition to other big problems in TFF too): both movies felt like TV episodes. IMO a ST film should have scope. It should, at the very least, feel bigger and take more risks than the TV episodes. When I saw both TFF and INS, I felt like I'd seen better ST episodes on television (Best of Both World in the cast of TFF at that time and many other TNG two-parters were far far superior to INS). That may be the biggest reason for me voting for INS as the worst TNG film. I really do think it's worse than NEM and it, along with VOY and ENT, is what led to NEM bombing at the box office instead of just doing poorly, like TFF did.
