1) Highlander 2- While Jack and Jill remains the worst movie I ever paid money to see, this one genuinely crushed my soul do to my love of the original and all the hype going into it. It basically sunk the franchise into permanent cable TV show and direct to DVD status forever after. Its so bad, even the people in charge of the IP just pretend it never happened. I was angry after seeing this and skipped the Adrian Paul show on its initial run due to how bad this was.
This was a while ago so I might have already responded to it, but I'll do it again either way: Highlander: The Source (the fifth and final movie) was somehow even worse than this.
Highlander is such a weird series. The first movie is great, the third and fourth movies are better than people remember, The Series is legitimately fantastic (I'd say it's even the high mark, like the Buffy series). Then you have all the complete assery the same company put out: Highlander 2 and 5 are two of the worst movies I've ever seen, while the Highlander: The Raven spinoff takes one of the show's best characters and ruins her immediately, giving us 20ish episodes of boredom before finally having a couple of good episodes right before the doors closed.
And even The Series, as good as it was, continuously struggled with producers and other folks trying to ruin it. Season 1 they tried to make it a Cop Drama, which were all the rage at the time, with the immortal protagonist helping the local police solve various cases and take out various other immortals who were murderers and whatnot. It was actually pretty rough at first, and for the first 12 episodes or so I could get someone wondering why the hell I talked them into watching this show. Then when it was clearly not working, they turned the reins over to this guy named David Abramowitz who totally fixed the series basically overnight, making it a more foggy and mystical show that focuses on the immortals themselves and their history / struggles. "Band of Brothers" in S1 (ep 13 I think) is the first good episode and it's pretty goddamn good. Show takes off from there.
Funny thing is, they tried again with the "Cop Show" bullshit a few years later with the Raven spinoff, and again it failed. Only this time there wasn't anyone there to bail it out. With a less-likable protagonist that show was DOA. They started copying the main show's formula in the last five or so episodes but it was too late.
Another thing they tried to do a lot of was "post-apocalypse" settings. They were obsessed with cop shows and post-apocalyptic shit. Just having everything be dark and run-down and dystopian, like the terrible second season of War of the Worlds in 1990. So they made a bunch of terrible choices toward the end of Highlander: The Series with the intent of moving the show to a dark dystopian setting going forward. The star of the show balked at all of this and said the fans would hate it, so they dropped the idea, but not before messing things up with The Devil coming to Earth and shit so they could LARP End of Days for a few episodes.
It's pretty clear the Highlander series has some incredibly talented people working for it while also having some terrible producers and "ideas people" behind it.