It's really hard to pick between Highlander II and Batman & Robin.
The other movies, bad as they were, just don't measure up in terms of sheer awfulness, all other things being equal.
Highlander II was something that just made no sense from start to finish. I can't imagine someone reading that script and greenlighting it.
It was full of scenes that were gratuitous, unnecessary, pointless, and embarrassing. Just really, genuinely, one of the worst films I have ever seen. If there were a button that could expunge that film from existence, you'd have to finally drag me away from it, I'd be pressing it over and over again with such alacrity.