Insurrection is the one that honestly pisses me off the most and turned me off of Star Trek for quite a while. It had a great concept and totally squanders it. Then I read the 'Making of' book and it pisses me off even more because it seemed like they allowed Stewart and Spiner to derail the whole thing behind-the-scenes.
I seem to recall reading the same thing in various articles. It only got worse with Nemesis.
With the TOS era films even the worst one came across as near-miss. I still believe there were a lot of good moments in The Final Frontier and the kernals of potential greatness lay there largely unpopped - All it needed was one more top-to-bottom rewrite of the script before heading into production.
"Generations" was the victim of novices attempting to write for the big-screen, but even that was really just another near-miss. "First Contact" was a homerun. "Insurrection" struck out at the plate, while, "Nemesis" was, for all intents and purpose, a forefeit where, in odd fashion, those inolved attempted to re-enact the highlights from a previous game played by a different lineup.
You couple that with the bush league effort of "Enterprise" on weekly television no wonder attendence went down and fans left in favor of the stadiums of other franchises (both new and old).