Differentiating between 'so bad it's good', where one (if so inclined) can play spot the plot hole, guess what's coming next, snigger at the bad acting...and just plain bad....
I'll politely differ about Starship Troopers (the original). Not quite as good as Robocop (same writer, same director, same kind of tone). Giant bugs. Lots of guns. Black humour. Annoying characters killed off. Love it.
For me, the worst candidates try to be worthy, important, significant.....and just end up boring.
The English Patient. As per the lead characters in the Seinfeld episode of the same name, I hated it. Lead characters totally unsympathetic, predictable plot, and yet it won applause by the bucketload. Why?!!! Made a transatlantic trip considerably duller.
Inception. Huge, steaming piles of dialogue consisting of nothing but plot exposition, for no other reason than the basic premise couldn't be explained properly otherwise. It got switched off.
There are probably quite a few others in the same vein but I've managed to not think of them just yet.....