Star Trek V: The Final Frontier
What can I say, it was directed by William Shatner. Unfortunately, in his Star Trek film, he asks the viewer to laugh at the characters instead of with the characters, the same as happens in the TNG movie Insurrection, for which I've reserved a special place in hell.
The less said about it, the better. But, then it has it's moments, too. Spock, McCoy and Kirk sitting around the campfire singing "row, row,row your boat" is something I wouldn't want to miss.
Not so very astonishingly, I don't have that much on The Final Frontier yet.