Worst Sequel? Maybe not objectively, but to me, it takes a lot to top Star Trek VII: The Rise of Remus. First, OK, we know that this was intended to be a Next Generation movie, and that idea got scrapped at the last minute. But seriously? With the principal cast in their 60s and 70s, the best idea you have for the final Trek movie is a big, explodey action flick? Even better, you spend half the movie building up the tension in the Romulan Empire as a Federation-supported attempt at earning the pacifistic Remans their freedom, then expect us to buy it when out of nowhere, the Remans say "screw it, we're blowing up their planet?" Cut me a break.
On top of that, the new uniform redesign was ludicrous, the very use of the Enterprise-A conflicted with established canon (seriously, you pulled the Enterprise-A out of mothballs at the beginning of STVI, ended it by saying it was being decommissioned, and then it's just back again with no good reason?) half the cast only played bit parts (though I must admit, I liked the Excelsior riding to the rescue. It'd been done before, but damn if they didn't do a good job this time, too.)... This was just a steaming pile, and they should have left well enough alone.