Between those two? Insurrection was a hit over the head that knocked it down. But Nemesis was the lethal blow that killed it.
The TNG movies tried too much to follow the TOS formula. Instead of utilizing the ensemble the way the series did, it tried to focus on leads and let the rest be support. This was it's key mistake. "Generations" had to do this because of the transition, having Shatner as Kirk. "First Contact" did it, but less so, because everyone of the ensemble had something good and fun. It also had to focus a bit on Picard by virtue of the story. But it was a bit more even.
"Insurrection" really made it a Picard/Data movie and the rest got less, and then while "Nemesis" gave the rest a little more dialogue, it never the less was about Picard and Data and their "evil alternates" as it where.
Honestly, knowing how much Stewart and Spiner had influence behind the scenes, I frankly blame their egos a bit for such.