It's really hard to choose which one was the worst. All of them suck, but there are some good parts to be found.
I like Generations purely for nostalgic reasons, and because of fan service. Kirk and Picard in the same movie, no matter how contrived and poorly handled, is still conceptually cool. The music is great, too.
I like First Contact because it had the best directing of any of the movies, by a large margin. Action Picard sucks, though, and the idea of a borg queen is fucking stupid.
I like Insurrection because it featured a number of nice character moments, like Geordi seeing a rainbow (echoing back to the second episode of the series) and the fact that it contained a moral dilemma (even if they handled it poorly)... The movie looked cheap as fuck, though.
I like Nemesis because...... Because...... Wait, I don't like anything about Nemesis. Except the deleted scenes.... and Chateau Picard. At least it looked bigger budget than the cheap ass looking Insurrection.
I guess Nemesis is the worst.