Despite being poor, their plot is firmly in the Trek Universe.
I really don't think searching for God or personal vendettas are firmly Trek plots...
Anyway, I voted for Nemesis. It was just all around mindless. The Final Frontier is bad for sure, but it at least the camaraderie of Kirk, Spock, and McCoy was essential to the plot.
Generations and Insurrection were pretty close to the top of the list as well.
As of this writing, and echoing Cooleddie's post, TSFS still doesn't have any votes. Could it be that mediocre of a film?
The Voyage Home showcased the crew out of their element and without their usual deus ex machina devices, which made for a wonderful story all around.