No matter what you may feel about the pleasures (or lack thereof) of Star Trek: The Next Generation the television show, I think it'd be fair to say that everyone can agree that the TNG movies were somewhat lacking, at best (With the exception of First Contact). What went wrong when scaling the series up to the big screen?
Following the finale of the television series, the cast and crew of the Enterprise-D (and then, Enterprise-E) went on to head up four movies: Generations, First Contact, Insurrection and Nemesis, with each one theoretically offering adventures beyond the scope of the television show and also allowing for character development that went further than what was possible in the relatively self-contained episode television format. The only problem was, three of those four movies seemed so uncertain and so uneven that it was if there hadn't been seven years of solid (and, okay, occasionally ropey) storytelling practice up until that point.
Generations is, I think, the best of the worst, if that makes sense. Unlike Insurrection or the truly horrible Nemesis, its flaws feel more the result of flop sweat and trying to keep too many balls in the air than anything else, and there's something almost forgivable about that; Generations is a very ambitious movie, and as such, I think it's almost okay that it fails to come together into a totally cohesive movie in the end. If nothing else, having to try and wrap up the legacy of the lead character from the original incarnation of Trek is almost too much alone to deal with, never mind trying to do that, give equal time to the regular cast and tell a story. Call this one a valiant failure, then.
First Contact, on the other hand, is a success on almost every level; it manages to both feel like a TNG story and fulfill the needs and tropes of an action movie at the same time, expanding to become a movie, as opposed to an overgrown television episode. It helps that the movie has both memorable villains and a plot that feels as if it's "large" enough to be worth a movie, if that makes sense; time traveling to ensure that humanity reaches the stars is a grand notion that feels like compelling enough be the central plot of any movie, not just something with the existing framework of a Star Trek, and that allows the movie to just have fun exploring that idea and taking it to dramatic places along the way to a conclusion we all knew was coming.
The same can't be said of Insurrection, which feels more like a step backwards. Tonally uneven - there are far too many attempts at comedy that just fall flat through execution, with script, direction and (oddly enough) music cues conspiring to flatten performances that are trying their best despite poor material. That kind of sums up the movie; there's not enough here to really make this an interesting piece, and the script is surprisingly weak considering the author (Michael Piller, who headed up the series during its prime), and as hard as the actors try, the movie never really comes to life in any real way. At best, this movie feels like a subpar episode of the series, only twice as long and in better definition.
Even that, though, is better than Nemesis, which just… feels wrong. The sense of trying too hard from Generations is back, but it's mixed with a feeling that none of the characters actually feel like themselves, and that the usual TNG narrative logic has been thrown out to be replaced with something that just tonally doesn't fit; it was clearly an attempt to reboot the franchise and inject some new energy, but the result is a sense of discomfort and awkwardness. Nemesis is a film that feels not just like a misstep, but a tumble down some stairs; a self-loathing Star Trek film that wishes that the franchise was something very different from what it actually is.
And with that, TNG essentially stopped. It continued in the spin-off novels and comics, of course, but as a canonical beast starring the actors, it was done. It's a really sad way to go out, and one that was undeserving of the series that not only reignited the Star Trek franchise but arguably made it more popular than ever.