I voted for Nemesis. I just remember how tired the cast looked and how tired I was at the whole proceeding when I watched it. I bought it on DVD and it improved slightly after subsequent viewings, though I still think it was such a misfire that it remains the least enjoyable of the TNG films for me.
Insurrection wasn't perfect. I think the forced comedy didn't work for the most part, though some jokes did. And while Insurrection felt a bit light it wasn't dealing with light subject matter and I thought the Son'a had potential, especially their cool warships. I preferred using them instead of Nemesis giving us a Romulan movie where the Romulans took a backseat to the Remans. I didn't hate the Remans, but I wish the Romulans had been front and center.
My TNG rankings:
1. First Contact